The Wall is a bad idea.
I'll explain. No physical barrier will keep a motivated person out if it is unattended. Incursion detection plus a responding security force is what actually keeps people out. Detection and response is crucial, adding a costly terrain feature that is a minor inconvenience that takes a few minutes to bypass is inconsequential.
A better idea.
The US/Mexico border is ~2000 miles long. Wireless coverage for that length is about 60-70%. Tiny solar powered observation points consisting of solar charged battery, wireless communication, daytime hires video & nighttime infrared could be ruggedly built for about $2000 each. Placing them quasi-randomly with an average spacing of ~600' would require ~20k units at a cost of ~$40 Mil.
Response teams would need to be placed within less than about one hour of any incursion point in zones, or around 40-50 response stations. Teams would need to be 4-5 people and staffed 24 hours a day totaling $2M manhours/year. A wage of $40/hr puts the border response labor in the ballpark of $100M per year. The labor for monitoring 20k live feeds is similar, requiring ~250 people @ 80 feeds/person. With modern computer video analysis and motion tracking the 80 feeds per person is reasonable. AI will probably do a better job than humans soon if not already. Regardless, when a team isn't responding, they should be monitoring, so monitoring feeds does not double the labor cost, but probably only increases it by 25%.
Conclusion.
A build cost of $200M plus an ongoing yearly cost of ~$150M is on the ballpark of what is needed to actually secure the southern land border of the US. Even if a wall were to be built, this cost would remain or the wall would not be effective. The wall would add almost no function if it were built. Some walls for densely populated areas or areas with the presence of roads or easily traversed terrain could give more response time or decrease incursion challenges, but they are primarily already built.
I think Trump and any capable person who thinks about it knows all of this. That leads me to the conclusion that Trump never actually intended to build the wall. I hope he is using it as a symbol only to draw attention to the border issue - one that the half of the US with double digit IQs can easily grasp. A possibility that I find disconcerting is the prospect that he is following the long tradition of politicians who carefully construct the appearance of addressing the border issue while intentionally not actually solving it.
Godspeed.
https://www.usatoday.com/borde....r-wall/us-mexico-int
https://www.deadcellzones.com/coverage-maps.html
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