Currently TACA, known by many as Take Another Chance Airline because of their tendency to cancel flights, will get you between San Francisco and BsAs for the least Pesos. Then there is LAn Chile, much better service and more reliable. LAN Chile/Peru will get you to LAX where you can catch a local Southwest flight for 79 Dólares to San Farncisco. LAN may even have service to S.F. via arrangement with a secondary carrier. However, be prepared to spend 31 hours or more to get there. For some reason, the old flight routes from South America to LA and San Francisco have been chopped up. You inevitably fly from BsAs to Santiago where everyone gets off the plane and waits 2 to 4 hours for another plane with the same fight number. And it's not uncommon to have to change planes once again in Lima where you end up waiting for yet another 757 with the same flight number to show up ready to continue the next leg of the flight. If there is a connecting flight from LAX to S.F., it usually requires you need to lounge around LAX for several hours. Anyway that has been my experience when trying to fly from Argentina to Vancouver-- which always has required I change planes in San Francisco. LAN Chile and TACA will definitely offer the best deal on prices (around U$S600+). But it takes stamina to be so frugal.
The other alternative is to fly Delta from BsAs to Atlanta and then Atlanta to San Francisco. It costs substantially more and unlike LAN, the service is less than acceptable in coach and economy. LAN tends to treat everyone as if they are in business class. Delta extorts U$S5 for every beer and cocktail. LAN and TACA pour them out for free. Delta will cost you about U$S1,200+, and there's always the chance they may force you to change planes at Salt Lake City International Airport which, when you are tired from having traveling companions sleep and drool on you in coach for 16 hours is like being forced to deal with a layover in a nursery school because of all of the children running around that particular airport.
United and American will blast you out of BsAs directly to New York or Miami, respectfully. If you get a flight to New York you can often find a discount coast-to-coast flight for around $200 to San Francisco because United just loves to offer last minute discounts to LA and San Francisco to fill every seat on their planes. Again, the cost will be more than TACA or LAN.
The guys who maintain this web site recommend an online discount travel site that may help you find you a less expensive flight. I forget the name of the site, but it offers great deals to Spain and other points in Europe and Asia from BsAs. I haven't found it much use for booking the long-haul from Argentina to the Pacific Coast. If you have any fly miles with Delta, United, or American I recommend you work on cashing them in just to upgrade from coach to business class. Your life will be more comfortable on the first two 8 or 10 hour flight segments.
Hey Damon - I just booked a flight home in june and surprisingly the cheapest was booking direct through the Continental Airlines site, you might want to check them out. I also ended up signing up for their card (which is part of a pretty crappy partnership ring) so that for a flight EZE - Vancouver / Mtl - EZE I'm getting 15,000 miles.
If you have an ISIC card (or can get your teacher to give you some sort of evidence that you are taking classes) go to Asatej, they can probably get you a good deal -- they are like Travel Cuts or STA of South America. There's one on Sta Fe & Puyreddon and one at Soler de Abadia.
Good luck -- I'll warn you, with the price of gas increasing, finding flights for less that 800 (including tax) is going to be pretty difficult.