It just depends on time of the year. Where do you see LA for. $375 per way? I'm looking at November to go down and all flights around $1,000 round trip and that's going on LATAM. We never have cheap flights from West Coast. If you can plan far ahead of time it's better but my last trip it was a few weeks out and was crazy expensive. But do a search from EZE to many west coast cities in December and you will easily see some $2,000 US round trip flights in coach.
I'm not sure when the OP is planning to come to the USA. Presumably around the holidays to visit family? It's always been crazy expensive when I've gone down around then. I usually always fly airlines from the USA and it's expensive.
I agree $3,000 a month is easily enough to live in Buenos Aires.You don't need to spend crazy amounts on rent. I do think $500 to $600 for a really nice place is going to be tough in Buenos Aires but outside of the city for sure you can do it.
Colombia IS a poor country but stuff there isn't as cheap as you think it would be. I find food to be expensive. Utilities are expensive compared to Buenos Aires. Electricity is expensive there. Argentina is cheap because of the crazy inflation if you have dollars you're golden.