The 10 billions estimate you mentioned came from CNN video. I know you forgot something.
Did they mentioned........... Labor, heavy equipments, pave road how to get to those borders, pave road along the border walls, purchase land from private owners, housings for the construction workers, facilities ( at least 300 ) where to make the precast cement wall, maintenance? Did they include those on their estimates? The answer is no, no, no, no, no and no.
BTW...... Here is the estimates per mile (2009) in building those roads. My SWAG minimum 2,500 miles.