You're usually better off exchanging your money for pesos in Mexico. With USD you get approx 11:1 in Mexico, but if you buy pesos in the US then you're only getting a rate of approx 9:1. I don't know if the cambio's take CAD. You should check for next time because if they do then you'll be at a more favorable rate and it will stretch your dollar a bit. Or you could check the rate to buy USD in Canada. It's possible that what you lose on the CAD-USD exchange in Canada can be made back on the USD-MXN exchange in Mexico.