It's an estimated $25 billion.
http://www.cairco.org/issues/remittances
http://www.nbcnews.com/news/latino/mexico-got-more-money-remittances-oil-revenues-2015-n510346
10% a year would net 2.5 billion for building the wall.
Sure, that number would gradually decline.
This is the real reason mexico is so angry.
Their economy depends on the money they send back from the us, while we pick up the tab for their citizens use of schools, medical, and entitlement programs.