The US Census has shown that workers with any kind of master's degree on average make about $400,000 more in their careers than those with just bachelor's degrees. The coveted MM (Masters of Music Education) also tends to help determine promotions of even tenure (in the case of college professors in music education). But before you just launch directly into a MM course, take time to look at the commitments you need to make.
It can cost about $20,000 to $30,000 (US) to get a Masters of Music Education degree. This may or may not include the cost of software, textbooks, application fees and transportation costs, depending on which school you go to for your MM. You need to see if your employer will help you with the costs. Some colleges also offer scholarships and other financial aid offers for graduate students.
You need to be sure that music education is where your passion lies. Despite the US Census reports, many schools are dropping music programs (although colleges aren't), and so the future of this career is shaky. If you are a music educator for public or private schools, than an MM will most likely not be a cost effective objective, unless you would rather teach in higher education.
Another career you could pursue with an MM is piano tuning, but there's not much call for that anymore. However, as long as there is one piano in the world, there will be a need for piano tuners. (Plus, it'll impress your friends!). But an MM will also greatly help you with getting taken seriously in your own profession.
Some holders of an MM relish being able to delve into their chosen passion and gain a new fire for music that helps in teaching their pupils. Some also like the MM to help them get papers or books published to help promote music in the schools. Just be sure this is the career for you before you invest in an MM.