Good comment, but it's made even funnier by the fact that a lot of the economic benefits of building the wall may accrue to Mexico. So not only will Mexico not pay, it's likely Donald will pay Mexico.

https://www.nytimes.com/2017/01/28/business/mexico-border-wall-trump.html?_r=0

Excerpt: "In another twist, money may flow to Mexicans or Mexican companies. Analysts say it is basically cost prohibitive to ship heavy rock or concrete more than 70 miles, or cement more than several hundred miles. That means manufacturers closest to the border may prove to be the most economical. That could be a big win for Cemex, Mexico’s largest cement manufacturer, which has a United States-based subsidiary that could bid for the project and several plants dotting the border, analysts note. The company could also potentially receive hard-to-trace subcontracts that even government agencies sometimes have a difficult time tracking."