First, this NYC job posting doesn't require a MPH- I don't have my degree yet, but I am qualified for that position. Next, I think that just happens to be an exceptionally low paying job. Take a look
here, which is a job at the RI Dept of Health. RI/Providence has significantly lower living costs than NYC, and the lowest salary range for that job is higher than the NYC one, AND it still doesn't require anything more than a Bachelor's degree. As to the last part of your statement...you justify the pay if this is what you really want to do. I have explored other career paths enough to know that PH is what I want to spend the rest of my life doing, and no other field will interest me to the same degree. So, I'll continue working my butt off to put away money so I can get my MPH without taking out excessive loans, and then I'll continue on a career path that may not be the most lucrative, but is one which I think will allow me to positively impact the lives of many, many people. That, at least, is how I am justifying it.