Most fuels in Mexico are subsidized to include aviation fuels. At least on the turbine side. But whether that equates to cheaper total price with the final bill depends on the FBO as there used to be additional "fees" for N reg aircraft. There was also a time when US drivers would head south to fill up but things changed in some areas near the border.
Also note, most of the rest of the world it is $/liter not $/gallon, so what seems like it is $2-$3 cheaper is actually more expensive as there are 3.78 liters in a gallon.