you sell cards here. i would charge quite a bit if it was stock, at least as much as you would get if they bought the card here.
you could offer a custom design ask for the graphic they want and make it. but that is a pain. i don't trust outside companies unless they plan on making thousands of cards. these small companies aren't usually worth it, or they are there to take them.
at the very least you'll need to make a contract stating exactly what they can do with the image, how many they can print etc. and any alterations would be cropping or adding to the image and not modifying the image itself. with a name and site mention on the back.
a getty calculator works pretty well. i think its a bit high, but i don't buy stock. you'll have to know how many they will be printing. you should also ask them their budget, it may be so low that you can just laugh it off.
---Mike Savad
http://www.MikeSavad.com