Very good estimate, but I noticed you did not take into account the high tech security equipment. e.g. cameras and detectors, in the mix ( they have those at the border between San Diego and Tijuana and Israel’s wall with Gaza as well ).
That is why a had such a big percentage for incidental features. I have done force protection construction for the armed forces. There are lots of aspects to this that would be cost items if I was going to speculate. Anti-tunneling cut off slurry walls below ground is something you would want to have in certain remote areas.
Where we have border construction, it has sometimes required the Corps of Engineers to build a road to get to an area, then provide protection from snipers.