Oh. South America is massive. And inter-travel between the countries is still mostly on the back burner with the exception of buses. I flew throughout South America since traveling by bus usually takes entire DAYS not just hours. (My friend took a bus for 72 hours from Buenos Aires to Lima) I happened to be on a tight timeframe, but if you aren't - the buses are actually super badass, clean and safe. They're like VIP, first class plane seats on the bus.
Anyway - you can imagine my surprise when I learned HOW EXPENSIVE flying is in South America, especially when you are traveling from country to country. (Intraflying tends to be a little cheaper) It doesn't matter how many sites you use - Orbitz, Skyscanner, Momoondo etc.. plane tickets are worth your soul down there. What you can do..and this is a bit of a specific hack - is to use Colombia's version of Ryanair... VIVA COLOMBIA! Their flights are super cheap. A $600USD flight from Lima - Buenos Aires on TAM would be $176USD on VIVA COLOMBIA. The only catch is that you must purchase the flights in Colombia. Or have someone purchase it for you from there, be aware that they charge you up the ass for everything. Including baggage, ticket print outs..
KEEP IN MIND: Most South American countries require proof of exit/ return when you enter a country. Before you get on a flight for Colombia, for example, you MUST show some sort of proof to the staff that you have some sort of transportation/itinerary that you will leave Colombia as well.
So for you one way ticketers like myself..listen up. If you don't have any proof - they will make you BUY A TICKET AT THE AIRPORT. This tends to be very expensive. On a particular hairy situation, with 20 mins left before my flight, the staff REFUSED to let me check in without an exit ticket. Even if you check in online, they will still need to see your passport and ticket. Instead of paying for a $500 flight to some random city at the airport, I bought a ticket with my credit card on Orbitz.com with my phone. They have a free 24 hour cancellation period, and I never got charged. I showed the itinerary to the lady at the gate and she let me through.
Win. Win. Situation.