I know of a retired NM who devotes all his time to teaching, and is arguably a better coach than some GMs with busy schedules (alas, the NM also charges twice as much!).
I think I know who you are talking about, NM Dan Heisman? If so I agree this guy is an amazing teacher, heck I'd have tried to get lessons from him if they weren't so expensive. Still I try to listen to his one hour live show on ICC every week to learn a thing or two. Anyway disregard this comment if you were talking about someone else...
I agree though you don't have to be the best of the best to have a deep and worthwhile understanding of the game that you can then impart to others. Not to mention a lot of GM's make awful teachers as the ability to teach in a meaningful manner can be a different skillset all it's own.
Coaching fees generally vary from $15 to $100 per hour. It's often recommended to choose a coach rated at least 400 points above you. After that factors such as coaching style, playing style, and dedication to students may be more important.
I know of a retired NM who devotes all his time to teaching, and is arguably a better coach than some GMs with busy schedules (alas, the NM also charges twice as much!).