Yes, airport taxes change frequently in South America, and often must be paid in cash (usually local currency only; sometimes USD or euros accepted).

Happened to me at EZE and CCS. Was told at the time of ticketing that all departure taxes were included, but then I got to the airport and had to fork over more, in local currency, before they would let me clear exit immigration.

Moral of the story: always carry extra local currency with you in South America! Be sure not to spend it before you get to the airport.