To make a prefab concrete sheet about 35’ x 24’ at around 2’ thick would cost around 8k a sheet. The sheets alone would be about 2 million a mile. Probably about 4-5 million with labor and accessory items.
So if we tax Mexico at 10% of their remittances, we’d only get enough to build 500 miles or so of wall, a year.
Take a hell of a truck to haul that sucker.