(MONEY Magazine) – Give credit where it's due: Collectors can create value in almost anything. And these days a growing number have set their sights on old credit cards, shelling out $500 to $700 for pieces of expired plastic that most of us cut up and throw away. These card collectors already have their own group, the American Credit Piece Collectors Association (517-839-2026), headed by an enthusiastic president, Bill Wieland. "Credit cards are the rare coins of tomorrow," says Wieland. "Prices have gone from virtually nothing to very significant amounts." Significant? How's more than $3,000 for significant? Over the past few years, in fact, the value of some of those familiar rectangles, usually expired or from closed accounts, has more than tripled. Three years ago, for example, early purple-hued American Express cards from the 1960s sold for $30 to $40. Now, serious collectors and dealers will pay ) $100 to $150 for them. That is, of course, if you can find such buyers. Collecting credit cards -- and their predecessors, charge coins and charge-a-plates -- is still a new field of numismatics, or coin collecting: There are fewer than 800 credit-card collectors today, and most coin dealers have yet to pick up the trade. Still, you can buy cards in lots of one dozen or more for $2 to $3 apiece through classified ads in numismatics or antiques magazines. They also turn up at flea markets and estate sales. Not surprisingly, many of the most valuable cards are vintage. Original Diners Club cards from 1950 can be worth $1,500 today, says Lin Overholt, publisher of Card Trader, a monthly collectors newsletter ($12 a year; P.O. Box 8481, Madeira Beach, Fla. 33738). The earliest American Express cards, issued in 1958 on lightweight cardboard and bearing the expiration date 4/30/ 59, are worth $3,400 in mint condition -- which means, ideally, that the card was never signed. Even some recently issued cards are collectibles, prized either for their graphics or because of the issuer. For instance, a MasterCard or Visa issued in 1989 by Florida Federal Savings Bank, picturing the endangered manatee, is worth about $275 today, says Overholt, not least because that bank failed. Some collectors favor colorful cards from department stores, restaurants and hotels, while others prize gasoline company cards from the 1950s and '60s. Certain Texaco cards from the 1960s are now worth $125 in mint condition. That's small in comparison to the price put down at a June auction in Las Vegas sponsored by San Francisco auctioneers Butterfield & Butterfield. On the block was an American Express card from the early 1970s. The hammer dropped at a record $41,400. In this case, though, having a signature on the card was a plus: It was signed by Elvis Presley.