Gift for wedding volunteers

I have some wonderful friends and relatives who have volunteered their talents and services for my wedding as their gift to me and my fiance. My brother is an amazing pianist and is providing the music for the ceremony; my cousin is making the cake; and we have friends serving as vocalist and ceremony director. My fiance and I would like to give them monetary gifts to thank them for their contributions, but we're unsure of how much is appropriate. I know the rule of thumb for vendors is 15% of total bill, but with volunteers, I'm not sure how to determine that. We are on a budget, so we can't spend too much on gifts, but we want to make sure we convey how appreciative we are.