Yeah, try TAM. I got a return from Sao Paulo Guarulhos to Santiago de Chile for a little over £200 with about 2 weeks notice.
If you want to go overland there's a lot to see and do between those places.
Mendoza is 7 hours from Santiago, just over the Andes. From Mendoza you can get a bus to Buenos Aires in about 15 hours. From BA you can get one to Puerto Iguazu/Foz do Iguacu in about 14/15 hours. And probably the same length of time again to get to Sampa. If you're not adjusted to long bus rides it might be a killer but these are really cool places. If time allows I'd definately give those places a look. Specially Iguacu.