This might not be practical this time for the holidays if you have time constraints (or if you have kids etc), but I've found consistently cheap flights to many US destinations (LA, Miami, I think NYC) from São Paulo. If you are flexible with your dates, $390-ish USD seems to be a standard R/T price - I've done several searches over several months and it keeps coming up. I was tipped off about this by a user of this forum, who travelled to Atlanta this way.
I just searched the Google flight matrix for a 14 night trip to LA ('calendar of lowest fares') and the last available date for this price is on the 16th of December, for example.
Getting to São Paulo and back and ensuring adequate but not excessive transfer time etc is another matter, of course!