It would seem we already subsidize Mexico to an unseemly degree. Welfare on a grotesque scale. I don't agree that it is appropriate to blur the lines in this way, on the allegedly pragmatic grounds of prophylactic action.
Clearly Mexico, as one of the richest nations on the planet... has all the resources they need right now to do whatever is necessary to staunch illegal border excursions by their own illegally migrating populace of have-nots....sanctioned and abetted by their governments.
All of them. Past, present, and undoubtedly future. They will gladly set up 'make-work'infrastructure on their side of the border, with our money. But what it will actually do will only be like W's virtual fence, only be a cosmetic window-dressing...a pretext for the continued non-enforcement which is the actual marching order of the millenium. It will give the Mexican government a thin cover under which it can continue a pretense of cooperation.
Where we disagree is that I think it's better to have the facilities than not.
It's not fair that I must weed my neighbor's property. It costs me a bundle, but it's cheaper than leaving them there to send their seed across the line because I can clear the infestation to its natural boundaries. My neighbors won't do anything about them.