Tough question. I would say that if there is no demand for your services because of how much you are charging, I would consider it as a red flag. Also if you are charging a lot more than what your competition is charging, it might also be a bad sign. There is a wide variety of how much each trainer charges for their session. I usually check what other trainers are charging in the area where I operate and then I come up with a fee that is good enough for me and also stay competitive with others.