We just got back from a trip to Oregon - and not good news for you, unfortunately. Unless you're willing to drive 8 hrs, flying straight to SFO on a cheap ticket (like Virgin) isn't really an option - and you'd more than make up the cost difference in rental car/fuel fees. My best advice is to keep an eye on prices from discount airfare searches...try different combinations of airports. You might need to have 2 connections to get to your final destination - it sucks, but if money is #1 concern, then you need to be prepared for a budget journey.
We flew to Eugene and connected through Chicago then Portland on United.