I find steam cards curious, as they represent steam giving phantom wealth to players. The only input I can see from the "real" economy into the system are players stocking their wallets with real currency. Other players then take a small portion of this real currency per trade, while steam gets ~1 cent back per transaction, and pays devs ~1 cent.
Pessimistically, a scheme to get people to give steam their money, while paying devs a small sum and giving players some of the dev's art assets.
The net number of cards should normally be relatively constant to keep inflation/deflation under control without supervision. The average cost of a card, I guess, depends first on how many cards it takes to get a badge, and then the amount of cards in circulation (decided by the number of people who own the game), and then the age of said game.
Also, I suggest making the number of cards in your game odd, rather than even.