It appears 'the wall' is a complete waste of money as Mexican immigration is on the decline...

More Mexicans Leaving Than Coming to the U.S.

More Mexican immigrants have returned to Mexico from the U.S. than have migrated here since the end of the Great Recession, according to a new Pew Research Center analysis of newly available government data from both countries. The same data sources also show the overall flow of Mexican immigrants between the two countries is at its smallest since the 1990s, mostly due to a drop in the number of Mexican immigrants coming to the U.S.

http://www.pewhispanic.org/2015/11/19/more-mexicans-leaving-than-coming-to-the-u-s/
from 6.4 million in 2009