This is something I have seen a lot of photographers do, and none has ever made back the cost of printing the cards. I do graphic design and have had photographers hire me to design the cards and I even tell these paying clients that they don't want to do this because the market for them is so nonexistant. Many of them have done it anyway, thinking "I'm a good photographer, those other peple just sucked, mine will sell". They don't!
Here's the deal: The cost of printing is VERY high. Very. Unless you buy a lot of them (thousands of each card), and then the total cost is still a lot of money. People will not buy them if they have to pay more than $1 or $2 and even then sales will be small in quantity.