Changing your US dollars into pesos is no big deal. At O'Hare Airport in Chicago, they gave me 10.5 pesos per dollar, AND charged me a $3 service fee. When I landed at Guadalajara Airport, they gave me 11.5 pesos per dollar, with no service fee.
One day, when the exchange rate was 11.4 pesos per dollar (the rate changes every day), the local currency exchange gave me 11.3 per dollar, with no service fee.
Simple math. To convert pesos to dollars in your head:
1) Divide the number of pesos by ten.
2) Subtract 1/10 from what's left.
For example:
300 pesos, divided by 10, gives you 30. Subtract 1/10 of that (3) and you get $27. The EXACT answer is $26.08, but you will have lost a little when you exchanged the money.
Hope this helps, Vince