Just got back from Merida/Campeche
we rented a dodge stratus for 5 days. Including babyseat and ALI (extra liability insurance), it cost us about $300 US with taxes. The babyseat was 25 dollars for the 5 days and the insurance was about 14 dollars per day. The actual cost of the car was not bad. Taxes are significant however.
Also note that if caught speeding, the police will take your license plates away and you can reclaim after paying a fine. We narrowly escaped this fate (not sure if we would have just had to abandon the car on the highway or not?)
Lorna