It' s wise to determine the exchange rate prior to departure. Although it's safest to plan on paying cash for flying expenses, most airports accept credit cards for fuel. If you do pay in cash, don't expect exact change. Available airport data for U.S. pilots traveling to Mexico is minimal. Although you are already on an open flight plan, notify FSS prior to crossing the border.
Mexican airports require flight plans.
Unlike U.S. flight plans, you file them at the airport prior to departure or after arrival. Flights from controlled airports require a flight plan prior to departure. Flight plans are closed somewhat casually by checking in after landing. The cost of filing the plan used to involve a separate fee but now is included as a gasoline tax. The cost depends upon aircraft weight, number of engines, phase of the moon, and attitude of the agent.