There are a few "passes" for south-america: http://www.airtimetable.com/airpass_south_America.htm
I got the Mercosur Air Pass a few years ago and it was great. Good schedules, free changes, etc. As you pay for the miles you fly (http://www.aerolineas.com.ar/arg/mai...06&categoria=C) maybe it will be interesting for you to buy an open-jaw USA-GRU / SCL-USA and travel GRU-EZE-SCL with the Mercosur pass.
March is a great month for travelling there. It starts getting cold in April.