Employees are not paid what they are worth to a company. That is often impossible to determine anyway. What's a busboy worth to a restaurant? You can't operate one without them. But the wage is a function of supply and demand, and that market is distorted by millions of immigrants, both illegal and legal. And that market is only going to be more distorted in favor of employers if the Chamber of Commerce gets their way on "comprehensive immigration reform".
Of course they are. The free market automatically determines that, with the exception of distorting government controls like the minimum wage. Price controls, caps and floors on goods, services, or labor simply distort the natural value the market determines.
In a free market system, everything including labor is bought and sold for what it's worth to the buyer and the seller by definition. Otherwise, the transaction wouldn't happen.
Illegal immigrants are no different than counterfeit money printed for the richest Americans.