I agree with Harris, you have to check with your local market. Determine what others are charging as a baseline and adjust based on your experience. I’d suggest starting out slightly lower than the competition to build your business and then increase your rates periodically once you’re established. You could grandfather people into older rates as long as they keep training or start training again within 6-12 months. I don’t see charging more than your competition an issue if you can command the higher rate.
At some point you want to consider training fewer clients and charging more; trainers experience burnout fairly quickly if they are training too many clients or doing too many sessions. For an example, it’s better to do 20 sessions per week at $30/session than 30 sessions per week at $20/session. You’ll be able to provide a better experience for those you train – referrals aren’t solely based on how many clients you train, but how many clients you train well!