Well, most of my experience has been in the test prep world (where there is actually decent money to be made). I'm currently teaching/tutoring part time as I work on a dissertation.
From my experience, the major test prep companies would drool for an actual teacher who wants to teach for them. A lot of people applying to these big companies are bright students with high scores but no teaching experience (so these companies spend a lot of money training people how to teach). I didn't find it particularly enjoyable to work for one of these big companies, but it's much easier to go out on your own after you work for one of them for a year or so.
My guess is that you could make a fair amount of money doing SAT Subject testing for Math and Physics. The demand might depend on your area (unless you could find a way to teach online). If you want to get into the SAT world, you might want to eventually take the SAT again, and study for it. It's kinda funny doing that as an adult.
Feel free to PM sometime if you want to chat about particular companies.
