Opinions vary,depends on who does the building and how smart they are.
“Opinions vary” - snicker. Yeah, as to whether it would be $7 billion, or $10 billion, or $15 billion, or more. Tell me, do you really think - as impoverished as your intellect might be - that a single-strand barbed-wire fence strung along 6” diameter poles stuck in the ground every 3 to 4 feet is going to stop even the most effete of illegal immigrants?
No, I didn’t think so. That means that “cheap” is out of the question.