Who pays? and Who’s property would you seize to achieve such a dream?
How much property was seized by FedGov™ when the Interstate Highway System was built? Hmmmmm?
We all pay for our security.
You find one person that is unwilling to sell a small piece of border property for a proper fence and I will show you a person willing to sacrifice the lives of their countryman for selfish reasons.
But the USA controls all borderland. Period. National security overcomes all.