Gringo money is dwarfed by money Mexicans in the US (legal and illegal) send back to family in Mexico.
They send back some $30 billion. Tax that at 10 or 20% and it can pay for building the wall.
The estimate I read was nine billion U.S. dollars by Mexicans (both legal and illegal) in America sent back to Mexico. Probably close. Our fault since we hire them.