It never has been a huge bargin. Mexican labor is just a bit lower than Amerian labor, and they are willing to take on any job. The fact is America does not have enough people willing to do hard labor. Certainly American work ethic is one of the best, but most people would rather do something else. I am sure you disagree, but without Mexican labor, it would take 2 or 3 times longer to rebuild New Orleans.
I hire only citizens for my business (I'm in the light construction/landscape field), and I have many competitors who hire Mexicans. We are better, more efficient, and I'm quite successful. I think the value of an uneducated worker who can't communicate is way overvalued.
Just my personal, subjective experience.
You obviously don't spend much time around schools these days. As a teacher for 12 years, I can tell you without hesitation, that the work ethic of our young people has gone to hell. As a witness to local farms who CANNOT, no matter what they pay, find Americans who will work (they show up for a day, sweat a little putting levy gates in, and quit), I can verify that America's work ethic (as a whole) has gone downhill, and not just in labor jobs.
And as long as we continue to import our laborers, it's only going to get worse. How many NO residents are still without work and/or displaced? Why the heck are they not in NO helping with the rebuilding (not saying that none are...)? My understanding is that NO had a very high unemployement rate prior to the hurricane - so where are all of them? Still getting free housing from FEMA?