The threat of dumping somewhere between 11 million and more likely 40 million illegals back to Mexico is enough for Mexico to agree to pay for the fence. Their economy could not take that many being repatriated back to Mexico.
#1. If you are here illegally and we catch you, you go to jail.
#2. If you employ an illegal and we catch you, you both go to jail.
#3. If you rent/sell housing to an illegal and we catch you, you both go to jail.
The illegals will leave en masse.
When the Mexican government sees that coming, they'll get a hold of Trump to see when they can get started on building the wall.