This is a question we last tackled back in 2006, and what follows is an updated version of the answer supplied back then by Pat Lamb, author of The Slot Card Price Guide.
In 1982, the first slot card was produced by the Sands Casino in Atlantic City, in order to track the guest’s play while in the casino. Anyone savvy who plays the slots or tables has one, if not many slot cards (now more commonly known as players cards), which they use in each casino. These are like little pieces of plastic "art" and are very collectible, much like baseball cards. We currently list over 7,500 cards, with prices and rarity, in issue order, in our Slot Card Price Guide, starting with Las Vegas and ending with foreign cards. (Casino room keys are currently not as popular as slot cards, but are a great hobby in themselves. Due to security reasons, they tend to be a bit harder to obtain but their values don’t generally reach the astronomical levels that slot cards are capable of achieving.)
As a hobby, slot-card collecting has snowballed over the past decade or so, with several clubs springing up in many states. My husband and I started a club called PACK (People After Cards & Keys) about 13 years ago, which had over 200 members and each month kept collectors informed about new cards (75+ entries a month). Due to health problems, PACK lasted only 5 years, but there are several clubs which now take its place, including MSOCK (Mystic Society of Cards and Keys) in Mississippi and the Slot Cards and Room Keys Club, which has a meeting for 75-100 people annually in Las Vegas.
As a general rule, the older the card, the more valuable it becomes. Also, the less cards that are printed, the more the value increases. An example is the players card from the Dunes in Vegas (the former site of the current Bellagio resort), which is worth upwards of $500, since not many cards were issued before the casino closed. A Gold (high-roller) card from the Landmark is known to have sold for $1,000.
A word of caution: Be wary if buying or selling on eBay or from an unknown trader, as just with any other field of collectibles, there are scam artists out there, and others just looking to prey on the naive. In a more unusual scenario, we can cite the case of the Glory Hole Casino in Cripple Creek, Colorado. This was a very rare card, seeing the casino was open a short time, and the card was initially valued at $250. However, a box of blank (non-issued) cards was found by someone who decided to profit from them and the value dropped to $3 each, since the market was flooded (the cards issued to someone from the Glory Hole, which have a bar code on the reverse, are worth more than the blank cards that were being handed out like candy).
Common cards from the large casinos in Las Vegas are worth a dime, if that. The less issued, the greater the price. Common Indian Casino cards bring anywhere from $3-$30, with high-roller cards at the top of the price spectrum, since those are harder to otain. Race Track cards also can fetch a price of $3-$7 for a common card. Montana cards are the hardest to collect, since they are not used in the machine but are generally scanned as you enter or leave the casino. Montana issues one card per person, and even then it can take 2-3 weeks before they will make you the card, plus most often the cards are for locals only. In some instances, the casino even keeps the card for you inside the casino -- these are not allowed to leave the premises. Hence, you need to get crafty if you plan to add certain cards to your collection.
In terms of starting or enlarging a collection, the best place to buy slot cards is on eBay, although a lot of buying/trading also takes place on the slotcardbbs.com website, at conventions, via friendly email to other collectors, and even at yard sales. If you have cards for sale, a good reference point is the Slot Card Price Guide, or take a chance on eBay, where the cards will be seen by others who know their value. Here's a link to a site that lists various resources for slot card collectors.
