I was thinking about selling some of mine too.. I have a friend that wants to come to my house use my supplies and make cards and pay me for the supplies used... I have no idea how much to charge her..
It really depends on how simple/complex the card is. Have you tried to calculate in supplies how much it cost you to make said card? If you can do that, and try to sell the card for half of what it costs, that might be a good starting point depending on the cost, your area, etc.