What would be an appropriate amount for a musical commission?

My wife passed away recently, and I would like to ask an acquaintance, who is an excellent musician, to write a short musical piece in her honor. I’m thinking of something around 250 bars in length, or the equivalent of 8 stanzas of 4 lines in terms of poetry.

What would be an appropriate amount to offer him? I would like the amount to be enough to make it worth his while and not insulting to him or embarrassing to me, but I don’t want to go to the poor house over this, either!