As a freelancer, I built a trade show system 7 years ago and I was paid $10k for the planning and build.
My client says he loves this system and he's used it at around 50 trade shows since I built it. I'm not sure how much he charges the trade show to use it.
It has minimal bugs and requires little maintenance.
Would it be fair if I got royalties or some of the profits every time he uses it?