It all depends on where you live/local cost of living, the experience and skills of the trainer, the average market rate. Lots of factors. I would also check out your competition and see what they are charging so you can stay competitive.
Coach John Kane CPT
This is one of the fun parts of training. You charge what you want to. As all of the great answers have suggested, there are variables that will impact your success. Your education, your experience, your contacts will all influence your success in getting clients. Be reasonable, be fair. Typically $45-50/hr is a good starting point.
Fitness professionals should charge what they feel their services are worth. If you are actually worth what you charge, you will get it. Even in a market where the fee is high for the area, if you are worth it, you will get it. I usually ask potential clients what their budget is and then give them a program quote based on that budget.
While some clients have expressed uncertainty about my fees, as soon as they work with me a few times that is no longer an issue. And anyone that has been referred to me has already has an idea of my fees.