This brings up a good point.
Unless the U.S. plans to man this proposed wall in a style similar to East Germany, building a wall along the southwestern border is just a completely ineffective, inefficient, and expensive way to enact border control.
It would become the modern day Maginot Line. Anyone motivated enough to run north to get paid $2.00/hr under the table picking vegetables for 14 hours a day in the sun, and risk life and limb to do so, is probably more than willing to put in the extra effort to flank or tunnel our expensive exercise in political appeasement.
Nevermind that the Mexican government actually publishes and distributes a 'how-to' guide on illegal immigration in comic book format.
Source 1 Source 2 I'm sure they'll revise it to incorporate new ideas on how to get around a wall.