I recently did a similar experiment while waiting for a flight at Rome. I made two purchases within 5 minutes of each other at the same airport merchant...one purchase was charged to an American Express, the other to an MBNA Quantum MasterCard.

Wholesale exchange rate for 12/7/03 (source=oanda.com), $1 = 0.82 Euros

Amex Charge was 46.60 Euros
Amex billed me $57.96.
Effective exchange rate $1 = 0.80 Euros
Amex surcharge = 2% (0.82 / (46.6 / 57.96))

MBNA MC Charge 5.10 Euros
MBNA billed me $6.29.
Effective exchange rate $1 = 0.81 Euros
MBNA MC surcharge = 1.1% (0.82 / (5.1 / 6.29))

[This message has been edited by anthonyanthony (edited Dec 15, 2003).]