I read somewhere the monies being sent to Mexico is such a huge part of their economy that the Mexican Gov. will never agree to closed borders...and in fact deliberately offer ID’s etc. so people can go to the US... and Mexico has legal teams here in the US to protect it’s people, (cash cows), should any find themselves in legal difficuties.
It was $18 billion in 2005 according to the World Bank. I believe the remittances are worth more than tourism to their economy.
IIRC it is #2, behind drugs.
Second only to their oil revenue.