That all depends on what year. Usually the earlier the less, unless it is autographed or a memorabilia. Items like a Mickey Mantle rookie from 1952 is worth a lot of money, it sold for $50,000 in fact.
You can find prices on your cards for free on WikiAnswers. To determine a value of your card ask the question providing the following important information: The year the card was issued, the company that issued the card, the player, and the card number. Note that the year the card was issued is typically the year after the last year listed on the card.