Besides trying your hand at searching through the usual suspects (Adioso, matrix.itasoftware.com, google flights, hipmunk, etc.), which is going to give you the actual cost of flying, I've found the following:
It's possible to get a roundtrip on most major airlines between a few big South American cities and a few European cities for as low as 500 euros. In South America, you likely would fly from Sao Paulo, Rio or Buenos Aires. In Europe, Lisbon, Madrid, but also sometimes Amsterdam.
Last year, I managed to get a KLM flight between Amsterdam and Curacao for little over half the cost of a round trip. That sounds great, but getting off Curacao to the mainland is annoyingly expensive.
The only airline I'm aware of that regularly sells one way tickets between Europe and South America without charging an arm and a leg (but still charging noticeably more than half) is Royal Air Maroc.