Deposit $100 in a new checking account at American Security Bank in downtown Washington, and you will be charged 10 cents. Leave the money untouched for a month, however, and you will earn a 21-cent credit.

Take that same money down the street to Union First National Bank, open a new account and you will be handed a gift of Dalton china. But at the end of the month, unless you raise your balance to $200, you will be hit with a $3 service charge.

This is only one example of the confusion, technicalities and wide disparities in the cost of banking found in a Washington Post survey of metropolitan Washington's 15 largest banks.

Consider:

A checking account that costs as much as $94 a year at the Equitable Trust Bank of Maryland would be free at the Bank of Virginia.

A customer with an overdraft protection loan plan at the National Bank of Washington pays an interest rate of 11.5 percent. That same plan carries an 18 percent interest rate at the First American Bank of Maryland.

The price of a bounced check when a customer does not have overdraft protection is nearly twice as much at the First Virginia Bank as it is at the Citizens Bank and Trust Co. of Maryland.One charges $10 for a returned check, the other $6.

Savings accounts often pay the maximum allowable rate of interest - 5.25 percent. Not so at the American Security Bank or the National Bank of Washington where the rate on regular savings accounts is only 4.5 percent.

Fifteen banks that were asked to compute the interest they would pay on a $1,000 savings account deposit for one year gave answers ranging from $45.70 to $54.66.

The competition among financial institutions once was easy to define. There were the full-service bankers in silk suits peering out from barred teller cages in mausoleum-like buildings. And then there were the savings and loan officials, working out of storefront buildings and hustling customers to sign up for the $5-a-week savings plan. story continuing

The bankers and the S&L sniped at each other in Hatfield-and McCoy fashion. But that was about the extent of the difference. It was difficult to distinguish one S&L man from another. All bankers looked alike.

No more. Today, as the survey of Washington banks shows, banks in the same broad category are becoming as diverse and specialized as restaurants. As Willis Wheat, an Oklahoma banker says: "It is increasingly difficult for banks to be all things to all people, so they are becoming specialized."

This specialization, a trend that began in New York and in the last six years has spread to Washington and across the country, has made it more important than ever for comsumers to shop around to find the best deal.

tit is not a simple task.

Consumers who set off to track down banking bargains will need a calculator, a clear head and a sharp pencil. Thus armed, they might be able to separate the merits of one offer from another.

Maybe.

"It's very much like the supermarkets where the consumer is faced with gimmicks," said Gerald F. Hogan, legislative director for Consumers Federation of America.

Hogan thinks consumers can understand and benefit from the choices - if banks give them enough information. But he and other consumer leaders complain that the banks do not always do that. "People don't fully understand the penalties [imposed on some accounts] or how to compute them, because banks are not making adequate disclosures," said Ellen Broadman, an attorney for Consumers Union.

Consumers can understand clearly one aspect of the bank price changes. Services they once received free now wear price tags. Most notable among the vanishing free services is the checking account. The loss of free checking has left many customers less than thrilled.

"It's a matter of principle," said Linda Stevens, who lives in North Arlington with her husband, Larry. "I know the bank uses my money to make money because I always keep a reasonable balance in my account. And that burns me up. Why should I pay for checking when they are using my money? "

Mrs. Stevens found a free checking plan at a Virginia bank. Otherwise, she said, "you get nickel-and-dimed to death" with what appear to be modest checking charges.

Many banks routinely offer free checking for senior citizens and budget checking for consumers of any age who write only a few checks each month.

But the prolific check writer seeking a regular account will pay a premium price - unless he can find a special deal that fits his needs and his financial abilities.

One bright spot, bankers contend, is the savings that may be possible for consumers when automated banking equipment has been installed.

With electronic transactions replacing salaried clerks and expensive paper consumers may find that bank prices are more economical. That does not mean prices will go down, however. It just means they might not rise as quickly.

The proliferation of prices - which can result in a $94 price for a year's checking at one bank and free checking at another - is expected to continue as banks sharpen their strategy and refine their fee systems.

"Banks have been guilty of giving services away," said Pat Barry, a marketing specialist at Equitable Trust of Maryland. Her bank is the one with the $94 annual checking charge for a customer who writes 30 checks a month and maintains only $100 in savings.

That is an extreme example, she said, of her bank's checking charges.

The same person would pay far less at Equitable if he increased his average balance or switched to a plan better tailored to his check-writing habits, she said. With $200 in savings, instead of $100, the person would not pay any checking fee. But the Equitable account stops being free when the balance falls under $200.

At that point, the bank imposes a $1-a-month fee plus 20 cents for each check. That works out to $84 a year for the 30-check-a-month customer. Another $10 for printed checks pushes the tab to $94.

No other bank in the survey imposed penalties that high on customers failing the minimum-balance requirements for free checking.

A bank offering a marked contrast to Equitable is the Bank of Virginia. As long as a customer there maintains a mimimum balance of $100, his checking is free. Name-only check blanks also are free. If the customer's balance falls below the $100 mark at any time during the month, the Bank of Virginia imposes a flat fee of $2.

Checking also is free at Union First National Bank of Washington for those maintaining an average balance of $300 or more in an account. Should the balance fall below $300, the bank imposes a flat fee of $3.

There is no fee at Union First for individual deposits, however.

About one-third of the banks in the survey charge fees based on account analysis. They consider the balance and the activity as measured by the number of items moving through the account.

An item may be a check or a deposit.

Charging for deposits is a practice that sometimes startles new customers. "You mean I have to pay you to put my money in your bank? " intoned one young woman when the rule was explained to her.

Riggs National Bank, the largest of all area banks, requires a minimum average balance of $500 before a customer gets free checking. When the balance falls below$500, the bank charges 75 cents a month plus 10 cents for each item. That price is offset in part by the 25-cent credit that Riggs offers on each $100 average balance in the account.

For a customer who writes 30 checks, makes four deposits and maintains an average balance of $100, the monthly fee at Riggs would come to $3.90. That works out to $48 for the year. Name-only check forms are free.

Next door in downtown Washington, at American Security, 200 check blanks start at $4.72. There is no minimum balance rule that would provide automatic free checking. The bank charges 10 cents for each check and each deposit.

That is possible, since the new specialization practiced by banks is based on the concept of embracing certain customers while rejecting others.

For instance, a bank intent on capturing business accounts may design rates to lure them while offering little for small consumer customers. Another bank may court the small consumer customers. Another bank may court the small consumer and snab the businessman.

"Anyone can profile the segments of the market they want to go to," said banker Wheat.

That kind of marketing strategy is of course, not limited to banks. Retailers have practiced it for years. But the degree to which banks now are specializing appears to be an expanding phenomenon.

Customers with large balances ultimately may beat those dime fees, an American Security official said, because the bank pays a 21-cent credit toward each $100 average balance in the account.

Banks also have added a twist to their specializing. As they structure prices to weed out customers they do not want, they are ending the give aways. And they are imposing new prices that they say more accurately reflect the cost incurred in providing those services.

Now there are service charges on savings accounts at some banks when the balance drops below the minimum level. American Security Bank, for example, charges $1 a month when a regular savings account falls under the $200 mark. That $1 charge is necessary to offset the cost to the bank in handling a small account, an official said.

It is the checking account, however, that has undergone the most profound change.

The idea used to be to draw the customer in with free checking and then interest him in other services such as savings and loans," said Wheat. But even though some free checking survives, it is fast becoming "as obsolete as a dinosaur," he said.

William W. Sihler, the A. J. Morris professor of business administration at Darden School at the University of Virginia, offers this illustration of bankers breaking apart services and sticking on individual prices:

The all-you-can-eat buffet is out, he said, and a la carte pricing is in.

Washington area banks are moving in step with their colleagues around the country in the pricing parade.

"We're not ahead of the pack and we're certainly not at the rear," said David S. Smith Jr., director of marketing and business development at National Savings & Trust.

Price trends generally are established by the 50 big bank industry leaders, most of which are clustered in the New York area, he said. What those banks do is quickly followed by the 1,000 moderate-sized metropolitan banks, if they agree with the logic of the move.

Finally, in a fashion that is often slow and erratic, the trend reaches the 12,000-plus banks in the country with $100 million or less in deposits.

Now that this group of banks is adopting price specialization and imposing higher service charges, the changes are becoming visible to the average consumer.

Bankers said the present pricing was dictated by a variety of forces. One was the tripling of oil prices six years ago and the subsequent shakeup of the international money market. Suddenly foreign banks began to invade U.S. domestic markets and to compete for the lucrative loan business.

"Banks in the U.S. used to make money on loans and give their services free," said Sihler. Now they earn less from loans, he said, so they restructured prices for most services.

More sophisticated comptrollers at private companies also get credit for today's bank prices.

"A major portion of the banking community relied on corporate accounts for a long time," banker Smith said. But comptrollers now are so shrewd in putting excess funds to work - and thereby keeping their companies' bank accounts to a minimum - that banks have less money for their investments.

Even the regulations that govern the financial institutions have contributed to today's prices, banking analysts said.

Savings and loan institutions, for instance, can pay 5.5 percent on regular savings accounts. Banks cannot pay more than 5.25 percent.

Smith said a bank might conclude that it would be unable to expand its customer base significantly - and therefore its profits - by paying the maximum ceiling, since it would never match the S&L ceiling. One result, he said, could be the bank's decision to pay less than the maximum.

Consumer leaders tend to agree that regulations like that one cost consumers money by inhibiting competition. "We'd like to see the ceiling on the interest go," said a Consumers Union attorney.

Another development that has helped drive up prices is the anticipation that Congress will legalize true interest-bearing checking accounts at banks. Now the practice is allowed in New England and New York.

Federal rules now permit banks to establish savings-to-checking transfers, but even that will not be allowed after this year. A federal court recently ruled the practice illegal. CAPTION: Chart 1, OVERDRAFT LOANS, The Washington Post; Chart 2, SAVINGS; Chart 3, RETURNED CHECK CHARGES, The Washington Post; Chart 4, Checking Charges, The Washington Post