Mexico's GDP is a bit over $1.26 trillion dollars. Remittances are far less than 40% of that.
$24.8 billion last year which is more than they got from oil. 10% of that would be $2.4 billion so in 5 years the wall is paid for. I like it.
As for the exact extent that our porous border helps Mexico. It is certainly to the tune of tens if not hundreds of billions of dollars annually.
The United States acts as a pressure valve for Mexico allowing for their least productive and least educated people an opportunity to earn better than average Mexican wages in the U.S. and send that money back. Every illegal here is not only a potential income source for Mexico, it is also one less mouth they are required to feed. It is a double savings to them.