We met with the Rabbi regarding our August wedding.   We are traveling back "home" for wedding.  We are having one of our Rabbi teachers from day school marry us.   He is a mohel but also does some weddings.  He said his fee is by donation.  His wife and him will be at reception by choice bc it is like a reunion of HS former student for him.   He is not affiliated with a specific temple.  
What is an average donation?
Thanks.