Yes! Insurance is the big expense. A few companies include Insurance in the rate. America Car Rental is one of them. Another hidden cost is damage. Carefully inspect and photograph every bit of the vehicle with an employee in the picture. Some companies offer glass and tire insurance too. Prices vary wildly. Good thing you are very smart to do some shopping. Print your agreement in case they "lose" your reservation.
There is no special problem about driving in Mexico.
You didn't say in which area near Cancun you will be driving. Depending on where you are there are little differences. Cancun is just like any big city in the US. But the streets are laid out in diagonals and there are quite a few more "glorietas" (circles) than square Playa del Carmen. Vehicles in the circle have the right of way.
Street is Calle and hopefully you can decode "Avenida".
Of course signage is in Spanish and the speed limits and distance is in metric. A Kilometer is 0.6 miles. (1/2 +10% of original)
Alto means Stop or yield. That white arrow under the stop sign indicates One Way.
This one is No Parking. In addition to writing a ticket they take a license plate from your car.
Speed Bump. (Very important)
28c is 82f.
The rate of exchange for pesos to dollars is about 18. That is pretty much 0.6 again! Obtain your pesos from an ATM attached to a BANK. Not a free range ATM out on the street, in a bar or store.