In your wierd, alternate universe. The adverse effect on the infrastructure,schools, hospitals, roads far outwrights the "wealth" they create. For example, IIRC one study (they quoted it on John & Ken) showed labor was 5 cents on an average head of lettuce.
By driving down wages or forcing those that follow hiring laws out of business they create a perpetual underclass that takes much, much more than it gives.
Immigration enforcement is 1/2 of the solution. We need to see employers do hard prison time. Then watch the immigration slow down to a crawl.
The MMP was a great idea also -- we need it across the entire southern border.
Much gentler measures would do the job. What about making employers liable for underpaying the illegal workers and for the school/medical costs?
One could make the just amnesty - you report to the immigration and you get the green card plus nice compensation and bonus with employer reimbursing the state?
The illegal immigration would stop even faster!