Actually, jobs outsourced to Mexico through NAFTA are now being sent to China - which underbids the Mexicans in the cheap labor department.
The best labor rate for corporations: slave labor.
Free market disposable labor is cheaper. Slaves you need to feed well, house, make sure that they get enough rest, and care for them when they get sick. With commodity labor, burned out or disabled workers can be tossed out and replaced like a tire in a car.