Well, actually there is already something like that. To hire an H1-B, I believe you have to pay her more than the national average in that category of worker. So I guess there are ways to cheat that a little, but I am not sure how big an impact this has. I recently got a job and have many friend that are looking for jobs or just found one (some H1B's some not). It seems the companies don't care much about the fine details of the salaries once they find the skills they wants.
Here is the exact text from department of labor.
"Employers must attest to the Department of Labor that they will pay wages to the H-1B nonimmigrant workers that are at least equal to the actual wage paid by the employer to other workers with similar experience and qualifications for the job in question, or the prevailing wage for the occupation in the area of intended employment â" whichever is greater. "