We've rented a number of times, always with the full boat comp and liability insurance. We never bother with the personal insurance, as we have insurance that covers us both for medical and theft when we travel. Costs have run from $14us to $28us per day depending on what we have rented. I usually figure whatever the daily rental fee, double it and you will have the total. Make sure you get the details of the insurance cost worked out before you leave with the car. If it seems way too high, gather your stuff up and start to walk out. Usually the rate comes down before you can get out the door. If you are renting from an international company (Hertz, Avis, Thrifty etc) call their Mexican help-line before you leave the states, and get a toll-free number from them to use in Mexico. Asking the people at the counter to call the number to hash out a peoblem will almost always eliminate any problem you may have.
Your credit card insurance will work in Mexico, at least mine will. Problem is you have to have enough room on your card to pay for a totaled car, then hash it out with your credit card company when you get home. Not worth the hassle. Also, when you turn the car back in, they can claim chicken-**** minor damage if you get the insurance with a deductible. If you have the full coverage they usually only check to see if the spare tire is there and the majority of the car is in one piece. Any damage that occurs, they blame on the next un-insured person that rents it. Make sure you get a signed receipt when you pay with your credit card, and check your statement as soon as you get home. About half the time you get hosed on a bit on the exchange rate, but there's not much you can do about that. Car rental place figures your quote at one rate, and the credit card company figures it at another. 99% of the time the difference is not in your favor.
A few words of advice. Watch out for the gas station scams, and bone up on your Mexican driving rules. I think the locogringo.com forum has compiled a list of do's and dont's on their site. Keep your driving at night to a minimum and watch out for topes (speed bumps). You can actually catch air under all four wheels with a 20 year old VW bug.