It depends on where you work. I can tell you that when I landed my Houston Master's, it was worth an additional $5,200.00. My license is not active though. The owner of the company that I work for has his own license. I would say that in Houston that the license is worth between $12,000.00 to $20,000.00 a year above your base salary. I can't answer for other markets.
Doc HMEL #688