How much to donate to Rev for a burial of ashes?

My father died last month. There was memorial service and he was later cremated. We are now burying the ashes at the family plot. A reverend local to where my parents lived is going to come deliver a service graveside and my mom is wondering how much of a donation she should give as it is about a 45 min drive to the burial site and she wants to compensate him for his time.

Any idea what would be fair? Thanks.