There have been regular sub-$600 fares to Brazil, yet Buenos Aires seldom drops under $900 in 2015. The end of the year has some of the lowest fares I have seen for California to Buenos Aires AEP (secondary airport to EZE) with United fares under $800 for some dates in December and around $817 for more dates in November and December.
Apparently, there were $600 Copa fares to Buenos Aires EZE from LAX. They show up on Google Flights at $599 today, but price $1,300 to $1,500 when I follow through the links.
Basically, United Airlines flies you to Sao Paulo GRU Brazil, where you pick up TAM or Argentinas Aerolineas for the flight to EZE or AEP in Buenos Aires.
United Airlines/Argentinas Aerolineas $771.53
San Francisco SFO – Houston IAH – Sao Paulo GRU, Brazil – Buenos Aires AEP
Tuesday December 1 – Tuesday December 8
United Airlines/TAM $816.53
San Francisco SFO – Chicago ORD – Sao Paulo GRU, Brazil – Buenos Aires AEP
Wednesday November 11 – Sunday November 15