It would be really inexpensive if we had illegals build it before we gave them their walking papers.Somebody was running a few terms ago and laid out a plan for a double lined fence with motion detectors in between. I can't remember who but I do remember reading it and it really wasn't that expensive. So you'd have essientially a prison style fence, a few hundred feet of cleared land and then another prison styled fence. If anything enters the no-mans land section an alarm goes off, the cameras come on and border patrol has time to get there.
It still doesn't solve the problem of bordering a poor and corrupt nation. Maybe we should just annex Mexico and divide their state-owned oil company into chucks and sell it to the highest bidder. It seems like the citizens would support that, but who knows.
You gotta give it to Trump though. He's skillfully used this issue to set himself apart from a crowded field and has really hit a nerve with a lot of people.