It only took a couple of days to realize that the biggest single obstacle I faced in trying to tap the enormous potential of my new home computer wasn't my ignorance of computer languages or my lack of imagination.

Nope. More mundane than that. It was my personal checking account.

The other day provided a case in point. I forked over $299 for a gizmo that will enable me to hook up my Apple II Plus with other computers over the telephone. That way, I'll be able to speak to you from experience about the various data bank services.

But sometimes, when it comes to owning a full-sized home computer, it seems as if the costs never end. Let me give you a peek at my checkbook to demonstrate.

Late last year, I bought an Apple Family System for $1,950 plus tax. In addition to the computer, that package included a disc drive (something like a tape recorder that uses small plastic discs instead of tape and is linked to the computer's memory), a handful of programs (discs containing prerecorded information that the computer uses to perform certain tasks, like balancing my checkbook or enabling me to use it as a typewriter), a gadget that converted my home TV into a computer screen, some game-playing equipment, plus various manuals and cables.

At the same time, I also spent another $150 or so on an instructional program for BASIC (one of the easier computer languages), some books on my Apple and a box of discs so I could save all the great wonders I was going to create.

A couple of days later, my wife ordered a business program called VisiCalc for her home business. Cost: $163.

Then, a month later, we forked out $124.64 for several educational programs for the children.

That was followed by: $31.15 for another educational program; $168 for a special video monitor (thereby freeing up the color TV for its conventional duty); $630 for a printer, and $34.84 for yet another educational program. All of that was capped by my most recent purchase--the telephone gadget, called a modem--which permits the computer to converse over phone lines with other computers and data banks.

Grand total for five months of ownership: $3,641.78.

But with my new modem, I'll be able to plug into The Source, a McLean-based national databank service, or CompuServe, which operates out of Ohio, or Dialog, a California company.

Provided I've got the money.

The one-time hook-up fee for The Source (local: 734-7500; outside Washington area: 800-336-3366) is $100. CompuServe (800-848-8990), on the other hand, has much lower hook-up fees ($9.95 if you have a personal computer with a memory, like mine, $19.95 if you have a "dumb" terminal, one which generally needs to be linked to another computer to perform any functions). Those fees will entitle me to an account number and password so that nobody can sign on as me and run up an hourly use bill that I have to pay.

A typical monthly bill for The Source is $30-$35, including the minimum $10 monthly fee. Peak hourly rates, which occur during workdays, are $18, while evenings and weekend charges run $5.75 an hour. The rates drop to $4.25 a hour from midnight to dawn. CompuServe's peak rate is $22.50 an hour while its offpeak charge is $5 an hour. A CompuServe spokesman declined to estimate a typical monthly bill.

Dialog is in a somewhat different class because its databank includes vast informational archives. Accordingly, it is most frequently used by businesses and researchers. While it charges no hook-up fee, its hourly rates more than make up for it, ranging anywhere from $25 to $300 an hour, depending on which archive you're searching. Not something that most of us would want to take advantage of.

For Dialog and most other out-of-the-area data banks, there is an additional cost to local users: you must pay for the long-distance call you make to tie into the system. Two specialized long-distance telecomputing services, Tymnet and Telenet, are available to link you up with your distant data base at cheaper-than-C&P rates. But even so, their hourly rates run in the $5-$10 range, thereby inflating the rates of the two out-of-state databank services accordingly.

As a footnote, it's worth noting that "dumb" terminals are far cheaper to buy than true computers. So, if you're just interested in linking to the data banks, it won't cost you anywhere near what it's cost me to date.

I'll be writing more about "dumb" terminals, inexpensive home computers, and what the various telecomputing services offer in future columns.