29 CFR 1.7 (b) If there has not been sufficient similar construction within the area in the past year to make a wage determination, wages paid on similar construction in surrounding counties may be considered, Provided That projects in metropolitan counties may not be used as a source of data for a wage determination in a rural county, and projects in rural counties may not be used as a source of data for a wage determination for a metropolitan county."
(the term "may" is permissive (i.e. Not legally binding) and I don't think that there have been "prevailing wage" national border wall projects across the sonoran desert in the last year, nor does David-Bacon apply to prison labor or the US military. IT isn't that difficult to trench down 6' and stand 20' pre-cast, reinforced concrete slabs on end, then backfill. Cast steel plates into the concrete and weld them together with 6" between for water and animal passage. Where we can't trench, razor-wire entanglement-topped and bottomed welded 100 lb. landing mats secured to posts and buddy we can put posts ANYWHERE. Look at freeway fences. Let's say we can only wall 60% of the border. That leaves 40% upon we can concentrate our primary forces, with rapid response units triggered by electronic observation posts on the walled/solid-fenced portions. Nothing is 100% effective but even 30% (and I believe your number to be Wa-a-a-y low) is better than 100% INeffective, which is where we are right now. Add to this, placing a Federal bounty on invaders rather than punishing those who detain them, and this problem would dry up in WEEKS, with or without a wall.
Ahem. Over-the-beach volume is relatively low NOW. The Coast Guard cannot keep up with those relatively low levels NOW. Most of the present illegal immigrant flow will simply move to over-the-beach. A massive increase in those numbers will do two things: (1) make your wall an expensive failure, and (2) make it impossible to do anything to correct that failure to your advantage. (News flash: when you mess up in a spectacularly expensive manner, no one's going to ask you how it should be corrected.)
How many more 9/11's is it going to take to illustrate that point to you?
You know, the only people who masturbate like wild monkeys over the prospect of another 9/11 are (1) islamist terrorists and (2) the illegal-immigration malcontents on FR.
and I believe your number to be Wa-a-a-y low
It stops another 10% from attempting to cross. (Those are the ones who are too stupid to think of finding another way across.)
The remainder simply go to over-the-beach. You still have about 90% of the present flow making it into the country.