If you think this amount is good enough for you and the Manager/trainer, then I’m sure it’s the right salary. You could also offer as incentive a % of the sales or training or memberships he/she will be selling each month. You can set up a scale which will help you and the manager/trainer refer to in order to figure out his/her monthly income.
Keep in mind that having one person doing both duties (manager and trainer) it could be hard in keeping up with both duties. A manager has a number of responsibilities that are different form those of a trainer and if you want to grow and run your business flawless, you will need to separate the two. The last thing you need is to have an employee who is overwhelmed and eventually burn out.