I've been doing something similar to this myself. Though i personally just make greyscale cards and print them off with a fold mark and stick them together.

If your planning on actually marketing this idea for money well you would need to get them professionally printed. Remember to go to your local printer and ask them about what "stock" papers/card they have as they are generally 1/10th the price of the HP papers (or other big brands).

Personally though I would opt to bypass the whole expense issue by presenting the idea in digital format. The best way (for the customer) to provide this to people would be on a CD at a convention or Via the web. You could then produce them in an editable format so people can adjust and manage the cards themselves rather than buy a deck of 2nd level spell cards and only really end up using 3 of them.

Also provided in a digital format the user can house rule their cards, print them off cheaply in greyscale or colour and manually select which cards they want.

Hope that helps :D