Avianca is double the price of LATAM for foreigners, and all others (not sure what the local rate is), so it's definitely out.
The prices are pretty well fixed at $350 for r/t on Avianca, $160 on LATAM for foreigners and all other carriers (for everyone?), and $70 on LATAM for locals.
I guess requiring document id at booking is it, though. Well, that sucks.