Scenario.
Imagine an American auto body shop with, say, 10 undocumented workers earning $10/hour cash. The shop rate charged to the customer is, say, $100/hour. With the shop running at full tilt, that is, all 10 workers busy for a full 8 hour day, every day, the shop is pulling in $800/day PER EMPLOYEE! That's $8000/day gross income. The wages, however, amount to only $80/day per employee x 10 = $800/day gross wages. So that leaves the shop with $7200/day to play with. This is income that the 10 undocumented workers generated for the shop that wouldn't be there otherwise. This money is then distributed to others for Rent, Insurance, Utilities, Supplies, Materials, Advertising, etc. The $800/day paid to the workers is then distributed to others for Rent, Groceries, Clothing, Fuel, Utilities, Entertainment, etc. with some portion being sent back to Mexico, the lion's share stimulating the US economy domestically.
At least the monies sent to Mexico that would have been paid in income taxes are not going to support illegal wars in Afghanistan and Iraq.