There’s a small flaw in your logic: do you really think that building a big wall will reduce the number of illegal immigrants from 700,000 to 0 (ZERO) ??

Also, the maintenance cost and all the personnel working along the wall have to be included into the equation. Maybe some day it paids off, but that day surelly won’t be in five years. Maybe when that day comes immigration from the mexican border is the least of US’s problems, who knows…