We looked into renting a car a couple of weeks ago and found the rates to be absolutely extortionate. The thing that shocked us the most was the insurance liability. One of the major companies (I think it was Hertz) wanted to charge us over 2,000 pesos for two days and a night (for limited mileage) and puts a hold on between 16,000 and 20,000 pesos on your credit card until the car is returned in excellent condition. If you find a good deal, I'd be wary about checking your liability. In the end I just wasn't comfortable with it all, especially given the driving/roads here...