It is worth putting in your dates on some of the rental car Web sites or at Breezenet and seeing what somes up. Alamo is about $225 all in for the week going SAN-SFO in mid July, with National about ten bucks more.

You may also want to post on one or more of the rental car forums. When I was underage (lo these many long years ago), it used to be possible to find and misapproriate corporate discount codes which waive the extortionate $25/day surcharge for under-25 drivers. Of course, YMMV.