Just one final post as I have now left Argentina. MasterCard did indeed continue to rebate the difference in exchange rates. So the net cost was almost the same as Visa. I did make a second cash withdrawal via ATM (only 10,000 pesos) and the net coast (after my bank refunded the ATM fee) was on par with what Visa credit charges were. A month in BA for two people netted out to just about USD100 per day for two people. Eating out almost every night. Wine or beer every night.