A couple of observations...

An instructor of fretted instruments needs to have significantly more experience and ability than the level of skill they instruct. This to recognize tuning, technique(!), ear, and other concerns as they occur.

Those who teach for free, and I've encountered one as provided by a fundamentalist church for its flock, are usually worth what they charge, and students know it. To a much lesser extent, undercutting the marketplace will indicate a possible undercutting of the learning process. So, though it's okay to charge less, stay in the ballpark, offer "introductory" lessons, and play (teach) to your strengths.

When in doubt get professional help. ie, if you don't know squat about technique, as is common in guitarland, consult a pro so that you don't contribute to the proliferation of ineffecient technique. "Professional development" (I hate that expression for some reason) should be part of the plan anyway.

The only fee structure that has ever worked for me is to charge by the hour (sometimes, but rarely 1/2 hour). Charging by the level/ability/degree of difficulty is bogus. My time has the same value whether it's for a rank beginner, advanced player, or learning disabled. Raise your rates all at once at one time of the year. I only raise mine at September 1. Besides, it makes book-keeping so much easier.

Peace, Mooh.