Credit card values depend upon eight major factors:
Age: In general, older cards are worth more.
Condition: Many collectors prefer mint unused and unsigned cards, which were frequently originally acquired by a collector when issued for collection purposes. Any card that has been heavily used is usually severely degraded in value unless it is quite rare.
Rarity: Some types of cards are quite common and have little value, such as green Amercian Express cards or some of the ordinary Citibank cards. Because they were issued in such high numbers for so many years without change, many have been preserved. On the other hand, many of the thousands of types of affinity cards were issued in small numbers and may be extremely unusual, which increases their value.
Brand: Certain brands tend to carry premium prices, such as older purple American Express cards or relatively obscure gas & oil brands. For example, an initial AmEx paper card from 1959 recently sold for $1200, one of the highest prices ever recorded. Black AmEx cards also trade for a high premium.
Attractiveness: Many collectors prefer cards with interesting designs or pictures. Affinity or theme cards with a full color picture usually carry a premium over one with just a corporate design, a logo, or a reduced image.
Celebrity: Cards featuring celebrities may demand a premium, such as the Elvis Presley, Rolling Stones or KISS cards. Cards that actually belonged to a celebrity, with documentation, may carry very high premiums such as the recent sale of 8 department store cards that belonged to actress Katherine Hepburn.
Landmark: The relatively few cards that established industry landmarks are worth more. An example is the very first Visa card issued.
Extras: In a few cases, other items are available along with a card, such as original mailing envelope, card carrier, and so forth. This always increases the value of the card.
It is impossible to give a valuation range for specific types of cards. The best advice for those wishing to value a card is to review the ACCCS album entitled "Selected Sales Transactions." It contains over a thousand recent examples of sales with image, date, and price of transaction. Most are from ebay auctions. Since there is no dealer of credit cards and no fixed price list anywhere, all trades of collectible cards have negotiable prices, either via auction or through one-on-one transactions.
The club has several quite knowledgable collectors who may be able to advise other members about the value of particular cards. Since most collectors specialize in some sub-set, members seeking advice should contact the club president to get a recommendation.