I am saying the wall is not as expensive as "not having the wall". If increased security knocks that $113 Billion Dollar number down by more than its cost ($21 Billion) doesn't it make sense from a financial perspective?
Also, we are far from isolating, I have lived in three of the most diverse states in America, we are ok and that's not going to change anytime soon.
No one is banning immigration, America has always had a yearly target for immigration. It's not going to stop. Working against undocumented immigration is not stopping immigration.