If you are a PPL instructor and unpaid chances are that your student will still pay the same club dual rate for instruction.
Belowradar, it is just as well with you being a freelance that schools dont reduce their prices when having the benefit of unpaid PPL instructors. If they did the students might choose the "cheaper " option. That would be at the expense of people like you.
I think the proper ethical position is to keep the prices at the same level where it is based on the value of the outcome of the service, not the cost of providing it.