sounds like you are teaching in Canada?
I charge $38/hour in Canada. If you are part of a Registered Music Teacher's Association, there is a minimum fee that they recommend teachers charge in order to keep up a standard.
I believe if you don't have an ARCT or any other diploma, there is a recommended fee for student teachers. I can't remember what it is, but I think it differs with each teacher association depending where you live.
Perhaps, you could find out what others in your area are charging and charge a similar rate based on your experience? I believe experience counts as much as a diploma.