Maybe if you have some spare models he may have use for, offer him those instead. If he loves the hobby then he'd probably be up for that more than a high five and some cash, considering you guys are friends. You can also let him beat you the next few times you guys play against each other
It's hard to say what his painting would be worth though, without seeing examples. Some people charge $5 for
HQ's, some may charge in the hundreds if it's Golden Daemon quality.