I did holiday bazaar last october and cards were flying at $1 each. Depending on the simplicity of the cards I would say $1.50-$2.00 a card. I think I underestimated my price b/c while I did sell alot of single cards, I offered to do bulk orders and delver by 12/15. I didn't get any at the bazaar, but someone contacted me the next workday and asked for 80!
I would negotiate it with your friend b/4 you make them!
The good thing is, even at $1 each - it paid for the supplies and the stamp sets I needed to buy to make the cards (hee-hee!)