I do not know the pay scale, but it is not great. You are hired as a contractor. The reason I say this is because our ground school instructors are not employed by American Eagle. They are not on the pilots seniority list.
Now we do have ground instructors that are check airman that are Eagle pilots. Their primary job is ground school instruction, but they also work the sim too. Your problem is that you will have to get hired, bottom of the seniority list, then apply to the schoolhouse. Which is a good gig if you like that kind of work.
Caveat, I am a line pilot, not school house guy. I am on the outside looking in, but I think my info is fairly accurate.