Are you kidding? Its second only to oil money as Mexico's single largest source of revenue.
At $100 / day for illegal labor, it's not a lot. Some of my friends hire illegals which I strongly disagree with, but they have no choice because their business depends on a flexible supply of labor. Regulations and taxes would kill the business if they hired legally. So at $100 / day, that's 8 days of untaxed wages sent to Mexico. That's not a lot.