Appreciate that it may not fit your itinerary so well but if you have time look at a downtown location to rent from in San Francisco
We recently rented an SUV for 5 days San Francisco O'Farrel St to LAX from Hertz for $180 plus gas
The taxi from SFO to downtown is about $50 and takes about an hour so you should be able to make a big saving if you can spare a couple of hours
nickyboy