Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
The merchant pays the credit card company something like $.10 per transaction. I know American Express costs more than others to accept.

Actually, it's somewhere between 1-3% of the amount you charge for Visa and MasterCard, I don't know about AmEx but I imagine it's higher.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.