It's pretty simple. I brought $230 to the Travelex counter at the US Bank downtown, with the intention of trading for Mexican pesos. While in line, I checked the current rate - 13.015 pesos to the US dollar. At the window I was informed of the fees, $7.95 for the exchange, and a rate of 11.40. So, for my $230 and change, I received $2540 pesos. This represents a total loss of buying power of over 15%.
Alternately, had I pulled $2540 pesos out of an ATM once in Mexico, I would have received the 13 to 1 rate, minus a 3% currency fee, a $2 foreign ATM fee, and a likely $2 fee for use of the ATM. This means my $2540 pesos would have cost me approximately $205 US, or a savings of $25, representing an over 10% discount from Travelex.
This level of of fees for currency exchange is unacceptable and usurious. I will not do business with Travelex again, and you would be wise to do the math yourself as well.