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.
What the free market determines is the least amount the employer can pay to get the most productivity. The difference between that and the value of the work done is the profit or loss. Employers want to maximize profits, therefore want to minimize the amount they pay. How little they can pay is not determined by the value added, but by the balance between the supply of people capable and willing to do the job, and the demand for those people. Which is why the Chamber of Commerce and their cronies want to flood the country with cheap labor.