Don't be too quick to mock. 

Spending $25 Billion on a wall with the best part of a billion in annual operating costs will create a lot of construction jobs. That's a lot of F150s that are going to be bought. About 20% of the construction work force in the boarder zone is made up of undocumented workers, so they will have to bring in workers from elsewhere to make up for that, so lots of hostels and temporary accommodation will have to be build, even if they are all just on workfare...

Obviously it isn't a project that is economically useful, more like one of Mao's make work projects rather than say the TVA, but at least  those workers whose jobs are export dependent will have something to do once the trade barriers go up.

And all that for a 20% import tax paid by US citizens.

They might even find a nice snappy name for it all. Has anyone though of "Social Infrastructure"?