
How much does a regular round-trip flight to Los Angeles from Buenos Aires cost
Around 1,300USD with taxes and fees included. The rate may vary with different airlines and depends on the flight schedule you will choose, about 100USD to 200USD give or take.
This link for example shows a list of round-trip flights available on the specific dates I've chosen:
http://www.orbitz.com/App/ViewFlightSearchResults?retrieveParams=true&z=84af&r=8g&z=84b1&r=8i&lastPage=interstitial
You can also check out some promos and special rate discounts some airlines give during specific dates. Researching for different flight schedules before booking you flight is much better because airlines can change their rates especially on holidays.

