Prefab walls will not provide ramparts for the Border Patrol. Heck, you won't get more than a couple feet thickness, max--and I can build a breaching charge to get through that wall out of ordinary household chemcials.
All they have to do is put it out to bid. The construction companies will not come back with your kind of numbers.
I'm sure they will. After all, it's no skin off their nose if the government has to keep buying new sections of wall to replace damaged ones.
But when it doesn't work, because your specification $70 million dollars a mile or $14,000 a linear foot is way out of line.
When you're talking about ramparts for Border Patrol agents to cap off pepper spray or stingball rounds at illegals climbing the wall, you're talking about putting them in place on site, no prefab work is going to accomodate those. Pour all concrete on site, heavy construction required, lots of new roads required, lots of imported water required.
There's no way those highway barriers cost that much even trucked down to the borders.
Fine, go ahead and use them. The highway barriers I've seen won't stop any illegal aliens unless they're in wheelchairs.
The ramparts can be prefabed, too. They can be made to just attach to the top of the walls once they're set.
Those prefab walls are designed to stop a semi-truck. They're locked in place by steel beams.