Indie RPGs are notorious for never paying royalties.That's just the market reality. Get it up front, or half up-front, half on publication. Yes, it will add up quickly. That it not your problem. Art is a thing you budget for in RPGs. That he wants a lot of art does not create a moral obligation on your behalf. Cut him a bulk rate, if you must, but get the vast majority up front, because the odds of you seeing royalties are very very low.