Also look at brokers. A couple of years ago, I was looking to rent a car in SFO and drop off in PHX. Most brokers allowed free drop-off within California, quite a lot had free drop-offs between California and Nevada, but I only found one that allowed free drop-offs between Califoria, Nevada and Arizona.

Let me get check the details... It was at DERTour.de - 13 days in October 2008, Luxury (Cadillac DTS or similar) but we got a Cadillac SRX, price €566, rental company was Alamo.