After seeing a landscape scene that I liked, I asked my bil who is an artist, (who was with me at the time), if he would do me a painting inspired by it.
His work is abstract, he's only sold some items through art fairs but he is very talented imo & luckily I totally love the painting he's made us.
However, I said 'great, send us the bill' & later dh said similar, and bil said 'just assess it & send what you think it's worth', or similar.
Neither of us know what to do, dh says whatever we send, he'll be insulted! 
What would you do & how much?