Hi M K,
There are many factors to consider, but usually the rate is determined by your location, your experience, the number of people you train (single/doubles/group), what your competition charges and what your time is worth (after you have calculated your expenses, especially if you are a mobile trainer). I have seen trainers charging $25/hr and others charging $150+/hr (for singles). In my opinion, I would say that somewhere in the middle is probably where you would find what a really good trainer charges. I hope this gives you an idea about the hourly rates.