As a travel agent I always start looking for a rental car for a client by going to the Orbiz and Travelocity sites and searching for "car only". Both site will give you a matrix of all the agencies and cars available, and you can see who has the cheapest rate for the type of car you like. As I write this, looks like Fox is cheapest, $218 a week for a economy.

Also, be aware that with the high price of gas, the larger cars are not renting so well. So they have a lot of them available and you might be able to rent one for just a few bucks a week more than a compact.