If he builds it they will pay. Mexico will be losing that much and more probably. Mexicans send their money home.
A wall is not magically going to stop exportation of money. That part is a lot more complicated than most people realize.
First of all, most money sent from the US to Mexico is not sent by illegals but by legal immigrants and citizens.
It is also sent through private channels- direct mail or wire transfer like Western Union.
To impact expatriated money would be a lot bigger of an issue than just a wall. Hell, in reality a wall would barely make a dent in the illegal immigration issue- far less than people realize.
To impact money being sent would require a fundamental change in how we view these transactions, we would almost have to treat Mexico like a terrorist state and put blocks on it. Otherwise we would be impacting private citizen's transactions and making the currency the property of the government first.
This is one of those hyperbole issues that don't have a practical solution.