Been checking flights and found Best Travel to be very low on United between now and as late as February. I'm only searching from Atlanta but the connections offered are through Chicago and Washington Dulles. So, United probably offers a lot of originating cities these same connecting flights. I've found as low as $675 total departing as soon as mid January.
Also, you can land in SP stay as long as you want, then catch a $75 one way from SP to Rio on TAM Airlines, and return to US from Rio without any penalty(or start in Rio, return from Sao Paulo)- use multi city search.
RT under $700 is extremely cheap right now and makes even a few nights relatively cost effective. Carvinal is Feb 17-20 so try to miss that unless you want to pay triple for everything.
http://www.bt-store.com/