So, not surprisingly, Mexico is not going to pay for it, at least not for now. Republicans are right that the authority for the wall already exists in the 2006 fence act, and there’s probably loose change somewhere in the federal couch that can be used to start funding it. But a wall across the entire border is going to be expensive and billions will have to be appropriated for it. As a matter of enforcement, it’d be much better to use any marginal dollars available in the interior and for creation of a better exit-entry visa system. The worst case scenario here for us immigration hawks is that the wall becomes the sum total of immigration enforcement in the Republican mind and there’s huge fight over it, and then there is no real energy or resources left for interior enforcement, which is much more important. On all this, including how Mexico or Mexican immigrants can potentially be made to help fund the wall, I recommend this excellent Mark Krikorian piece.