Ok, so I compared the cheapest RT fares I could find in August for YYZ-CZM $364 and DFW-CZM $453, using USDs for both. Pretty unappealing routes with two stops each way for both tickets, and for you having to clear our I&C arriving and TSA leaving, but just to look at taxes...
YYZ-CZM $364.38 - $152.60 to the airline = $211.78 taxes & fees
DFW-CZM $453.48 - $254.00 to the airline = $199.48 taxes & fees
Not much difference in taxes & fees, but they do add up and yours seems higher when included with the US & Mexican taxes and fees. $112.58 goes to Mexican charges on both. You can check my figures on Matrix.