See what the cost range is in your area and fall in line with that. I agree that training 20 sessions a week for $30 is better than 30 sessions a week for $20 per session. If you aren’t on top of your game that isn’t a good thing.
Offer packages where the more sessions purchased, they get a reduced price per session. Instead of 10 sessions at $65 / session offer 20 sessions at $55 / session. Then you have more stability for you AND a longer time with them to be sure your clients are successful – and getting a nice deal too.