If a current client refers you to a new client, do you offer a finder fee or discount on their next invoice? If so, how do you calculate the fee?

In the past, I have always just comped about $100 of work time as I was still a start-up. Now I have a high profile coaching mentor who wants to refer me to her clients. She wants a $125 up front for each sign on and than $125 3 months later if they stay on. Does any of you do something like this?