I don't know about the airport's own lots, but most offsite start at $12/day (plus taxes in some jurisdictions), with the 7th day free at some. If you are not going to be a frequent parker, the Park SFO lot may be your best bet as they usually have a free day coupon on the webpage. My usual is Park'n'fly. They have a AAA rate. I refuse to use PCA. I can't give you any recommendations on the south side of the airport.