On December 20th, the government of Argentina will start charging a "reciprocity fee" for travelers entering the country through Buenos Aires international airport (Ezeiza). At this point, travelers entering Argentina by going overland or by ship are not required to pay this fee.
This fee only applies to citizens of the United States, Canada, and Australia. Each travelers has to pay the following amount:
United States: $131
This can be paid in U.S. Dollars, Argentine Pesos, travelers checks, or credit card. The payment of this fee is NOT for a visa, as travelers from the United States, Canada, and Australia do not need a visa if on vacation. You can see more information by going to the Argentine Ministry of Interior's site.