Retention Bonus for Personal Trainer: What is a fair rate?

I have two personal trainers that are working for me and I want to give them a retention bonus every time one of their clients re-signs for more personal training sessions. Can you give me some insight/advice on what is a fair rate to pay. When they sign up a new client, I give them 10%, so I am assuming the retention bonus will be less, since it will be residual.
Thanks,
Kisar