The cost of mainstream investing is coming down--not in a trickle, but in a rush.

You expect low prices from online brokers. Ameritrade, for one, is offering one month of free trading.

But who expected giant American Express Financial Services to offer free trades too?

At americanexpress.com/trade, you can buy and sell stocks and no-load mutual funds at no cost, if your account is worth $100,000 or more. Investors with accounts under $25,000 pay $14.95 per trade. In between, you can buy stocks free but will pay $14.95 to sell. (At AmEx, as with the other brokers mentioned here, there are limits and exceptions, so check.)

"Pure order execution is a commodity that costs very little," says Ruediger Adolf, AmEx's senior vice president. Adolf expects to make money on customers' cash balances, interest on their margin loans and personal investment advice (it costs $44.95 to talk to a broker) and by selling pricey financial-planning services.

The big full-service brokerage firms dismiss Adolf's argument. They believe they'll always get affluent clients who will pay for orders.

But people with big money have "play money," too, and they're increasingly investing it online. And young investors? To them, full-service firms look like their grandfathers' brokers.

To get with it, Morgan Stanley Dean Witter now offers online investing at $29.95 per trade for people who have at least $2,000 to open an account. At Merrill Lynch, it's $29.95 on accounts of $20,000 and up. At Salomon Smith Barney, it's $19.95, through an account you open with its merger partner, Citibank.

To look more like financial planners, the big firms have also moved toward "fee-based" brokerage. Instead of paying commissions per trade, clients pay an annual percentage of the assets in their accounts.

In return, they get unlimited stock trading (through a broker or online), plus access to all of the firm's financial services.

Fee-based accounts target people who trade stock portfolios worth about $100,000 and up. Here, too, costs are falling fast.

Merrill drew the latest line in the sand, with a minimum fee of $1,500 a year and a top of 1 percent. What will Smith Barney and PaineWebber do now, with a top of 1.5 percent?

Even now, these fees are negotiable--at PaineWebber, down to 0.75 percent. Financial planners who charge 1.5 percent to manage individual stock accounts will also find it hard to hold their price.

There's another form of fee-based brokerage--the "wrap account"--for people investing as little as $10,000 to $25,000. With wraps, brokers typically put your money into a mix of mutual funds. Instead of commissions, you pay a fee.

The listed wrap fees average 1.5 percent, says consultant John Payne of Cerulli Associates in Boston. But you can generally get a quarter-point off the listed charge. (You also pay the mutual funds' own management fees.)

If you have a full-service broker but don't trade much, stick with commissions. They're cheaper than fees.

At the online brokerage firms, fees are settling at $10 to $15 for a plain-vanilla market trade, says Kenneth Michal of the American Association of Individual Investors in Chicago. The lowest: Brown & Co., at $5.

Instead of cutting prices further, most online firms are working on better, more reliable service, Michal says. That's going to be important when the trading bubble bursts and the unprofitable brokers scramble to say alive.

David Pottruck, co-chief executive of Charles Schwab & Co., believes that all brokers, including those online, will eventually "tier" their pricing, giving better deals to the more profitable accounts.

Financial planners also will have to charge their better customers less, says Mark Hurley, president of Undiscovered Managers. His firm finds money managers for planners who run retirement accounts.

Today, people investing $250,000 and up generally pay 1 percent or more. Hurley expects that, within seven years, competition will slash that cost to no more than 0.4 percent.

Small investors with simple questions will probably be served by the Internet. Every week, there seem to be more free tools online for budgeting, debt reduction, financial education and investment planning.

To help you use these tools, storefront planners might spring up--say, the way H&R Block delivers income-tax services today.

"It's a golden age for consumers," Pottruck says. The tougher you are on costs, the faster they'll fall.