You might want to look into the collective bargaining agreement rates the university TAs near you command if you're in a university town. These are usually publicly available. Here, a master's or doctoral student earns $35-$45/hour plus benefits, and an undergrad about $30/hour. I'm in a large city with a high cost of living. If I were hiring someone, I'd probably have to pay an extra 10% or so to approximate the benefits.
You could also contact the university department of the subject you're interested in and ask for going tutoring rates.
