'Staggering'?
No offense, but, about five years ago the cost of building one mile of a 6 Lane Expressway in my good ole IL was approx $1,000,000.00. That's $189.94 per linear foot. If we can't build a 20ft high wall for that something is drastically wrong.
BTW, I'm in construction too, have been since 1970. Mechanical Engineering of Commercial buildings mostly now. Have also been a Project Manager and Chief Estimator - from 57 Floor Hi-Rises to schools. And IMO the structural aspects of building a wall are not all that daunting, especially with Pre-Cast Planks (not poured Concrete).
We spend $100 billion a year in Iraq. Our sourthern border is just as important to national security as Iraq.