The exchange rate fluctuates, and depending upon where you exchange your money you will get a different rate. Use this quick reference to see approximately how much your dollars are worth based on the posted exchange rate. To use it as a quick reference card, select a smaller font for the web browser and print this page.
The worst place to exchange your money is an airport exchange booth outside Mexico. The best place is most stores or restaurants in Cancun, where the default (unless posted otherwise) is usually 8.00 pesos / dollar. You do not need any pesos when you arrive, because there are money exchange places in the Cancun airport and taxi drivers will take dollars (although they will round up to the next dollar). You should convert a small amount to pesos for convenience in paying bus fares, vending machines, and stores with low posted exchange rates; or just for convenience as most prices are quoted in pesos.