If you want cheap and you're not afraid to adventure a bit, i'd use ModerCa. Turn right at the National/Alamo agency's before the toll booth and they're 25 meters on the left. I pay $30 a day with everything included for a compact (Honda Accord, Nissan Sentra, etc). Airport pickup/drop off included of course, I've had them not charge me for minor accidents/dings and generally go above and beyond what's expected. Especially surprising because the smaller ones have a reputation for being real hustlers.
As much as I've heard about/seen great deals with mainstream agencies it's been my first hand experience that by the time they add mandatory insurance (More than the price of the rental) + taxes (26%) on the base fare/insurance you're paying a small fortune. $18 a day means $50.
With Ramon at ModerCa $30 means $30.