(MONEY Magazine) – With errors turning up in as many as one out of two credit reports, you might think it would be worth paying $19.50 or more to see what the three major credit bureaus have on you in their files. And you may have received mail solicitations from some of the half-dozen or so outfits that have sprung up to provide this service. But think twice before biting. In many cases, you can get the same data on your own for less -- or for free. Typically, these companies offer a merged summary of your credit agency files, plus instructions on how to interpret the information and challenge it if it's wrong. They also dangle various added attractions. For example, if you purchase Credco's Confidential Credit summary for $24, you get access to a toll-free line manned by people who can answer your questions. Reports from QualiNet's Credit Image ($19.50), available in January, will include a narrative description of your credit history, in case you don't like reading tables. And early in 1993, Credit Watch plans to introduce a $29-a-year ''negative alert.'' This service not only summarizes files from all three agencies but notifies you if derogatory data is filed with one of them -- TRW Information Services. For most consumers, however, these extras are of marginal value and your basic credit history may be available for little or nothing from the credit bureaus. TRW (214-235-1200), for example, will send you one free report per calendar year (subsequent ones cost $7.50 each). Equifax (800-685-1111) charges only $8 a report. And though Trans Union (216-779-7200) still imposes fees of as much as $20, you can get the report for less if you live in one of the four states -- California, Louisiana, Maine and Montana -- that limit how much all the bureaus can charge. If you live in Maryland or Vermont, you can get your credit history for free. And no matter where you live, the agencies will send their reports at no charge if you are turned down for credit because of something in them.