We decided to rent a car during our second trip to Cancun. We started out by asking the concierge at the resort. They quoted us around $45 a day with no insurance. This was with Hertz - our preferred agency. Hertz is also about two blocks down the road which was an easily walking distance from the resort. We also inquired with Avis and it was a little more. I thought that this was a little high, plus was concerned about insurance. We've read about renting cars in foreign countries before and how insurance is recommended.
The resort provides Internet access for about $2 per 30 minutes. I logged onto the Hertz website and found that we could rent a mid-sized vehicle for about $42 a day including full coverage insurance. We booked it over the Internet and wrote down the confirmation information. I would highly recommend either calling the rental car agency or going online and doing it that way if you are comfortable with that. When we went to pick the car up, the agent told us that walk-ups were given the $45 per day amount which doesn't include any insurance at all. Before we took the car off the lot, a guy did a walk-thru with us and noted any defects in the car, including little scratches and dents that we wouldn't have noticed. I have the distinct feeling that if we had not purchased insurance we would be held accountable for any new ones that may be created by us.
Throughout our travels outside of Cancun, we were hit many a time with flying stones and rocks from stone trucks. There are also a lot of potholes in the roads. It is not smooth by any means. The laws in Mexico hold you responsible for any damage to the car if you don't purchase insurance. I hope that this helps anyone that wants to rent a car in Cancun!