You pay for a wall the same way you pay for a bridge. You tax or levy a toll on the trade that passes over it.
Stop sending money to terrorist countries and we will have the money for the wall. Let the Army Engineers build it and while it is being built have the Military on the borders. We are in a dire situation right now. It has to be done.
Maybe that’s what Trump say when he was in Laredo with all the traffic in and out of there and Mexico.