I used my Canadian Visa credit card In Cancun last week. For a 75$ fee to pay an excursion, I was charged $87.48 Canadian, so 1.16 Can for 1 US dollar in Mexico.
Using Visa to pay 62.12 Mexican Pesos, I was chaged $5.86 canadian on my credit card account. So one Can Dollars = 10.6 pesos using my credit card. At the Canadian Airport, I exchanged 20 dollars Canadian, they gave me 200 pesos, 1 for 10, the worst exchange rate I got so far, Viva Canada! In Cancun, the direct over the counter exchange rate was 11.50 pesos for one dollar Canadian. But in the bus, you pay 1 US Dollars, no change back, for a Bus fee of 9.50 pesos, hum... got scre.. there. At another place, they gave me 12.50 pesos for one US dollars, others where around 13 pesos for one dollar. They told me if you witdraw Mexican Pesos with Visa, it would cost me more, do not know why, did not tried it.
Normally when I use my Canadian Visa in USA, I get charged 2% over the US dollar rate. Today it would have been approximatly $1.00, plus .02 cent, plus .13 cent, total 1.15.
I draw some US money from an ATM but did not get the rate yet. On the ATM machine, it said that a 6.85% charge would be added to the withdraw, this does not include the 2% from VISA plus the US Dollar charge rate in Mexico, I presume it will be an addition of around 21.65% on top of the US dollar.