In my, admittedly limited, experience with Mexican casinos, slot cards are really cash cards. In the casinos where I've played, cash can't be put directly into a slot machine. The card they give you isn't a rewards/membership card; it's a cash card. The card must be loaded with funds (pesos in my case) like a pre-paid debit card, and the money moves from the card to the machine when you insert the card and back to the card when you remove the card. Cancun casinos may be different since they have primarily tourist clientele, but, if it does work that way in Cancun, don't lose the card. :-)

BTW, you can think of a peso these days like a U.S nickel. 20 to a U.S. dollar. 100 pesos is 5 dollars.