Currency conversion costs from bankrate.com:
However, your credit issuer or bank often charges an additional fee, usually 2 percent, which adds up to a 3-percent total charge on foreign purchases. Bankrate contacted several credit card issuers, and all refused to explain the reason for the charge.
Follow the link for a list of how much each company charges. Until the credit card companies compete on trying to serve customers well instead of trying to trick customers well such articles are extremely important. The companies have more resources to invest in tricking you than you have to try and find all of the tricks they use.