On short notice, Aerolineas Argentina seems to often have the cheapest roundtrip tickets like EZE-GRU-EZE for c. $150 including taxes. I usually just throw away the return ticket since the one-way is usually more expensive than the roundtrip. Unfortunately EZE-GRU doesn't seem to be as consistent in having low fares as the EZE-SCL sector traditionally does.

Gol has some decent pricings once in a while too. The alternative is to get a package deal from a local Argentine travel agency which seems to work at the last minutes often enough too.