Mexico has HUGE CITIES just south of the border that will die without cross-border trade. We have one city north of the border (actually a town, 40,000 people), Calexico (in California) that relies economically on Mexicans coming over (from Mexicali, in that case).
The Mexicans better be VERY CAREFUL about angering Trump. In two weeks, with a pen and phone, he can SEAL OFF THAT BORDER, at least to legal crossings.
Or charge everybody coming in $20 to pay for the wall. Every day that could add up.