Depends on several things.
1- your background
2- your client’s goals
3- how long they’d like to sign on for
4- how often they’d see you a week/month
5- whats the % the gym gets
6- what are other gyms in the area paying for PT wise
After all of this is factored in you need to make the sale and present it on their health, and the value they get by working with you. Be sure to sell on wellness and measurable results.