It depends on the work. In my state illegals working in construction make between $12 and $20 per hour--a competitive wage. Their employers come off fat because they can shove the external costs of the labor onto taxpayers.
The illegals put their kids into the public schools at no cost to them, and they use the hospitals and clinics for free or heavily subsidized medical care. They may live in cramped conditions, but are still far better off than they would be in Mexico. Compared to how they were living in the corrupt socialist hellhole they came from, they are in living in paradise here.
Still, when their position is finally secure (amnesty) you can expect them to demand with a vengeance that they be paid what fully legal residents would get paid, complete with benefits. Employers will just hire new undocumented illegal aliens to perform the work that the "legal" amnestos are demanding to be paid higher wages for. Then we'll not only be stuck with the amnestos, but with a new subculture of Mexican workers. Meanwhile all the children of the amnestos will remain to become a permanent part of the socialist left.
And you know this how?
You don't run a real business do you, but pretend you do on FR, IMO.