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?
If you can create a 2.0 with substantial improvements, then you might have an opportunity to shop it around and rent it out yourself. But you'll have to do all of that at your own expense.
1) normally, they would explicitly say they own the IP to what you build for them
2) it sounds like they did not in this case, but you also did not explicitly say you would charge royalties
3) I don't know what the law says, but the normal way these things work is that if they paid you up front to make it, they own it, and it would give them a lot of legitimate grounds to feel ill-used if you were to find some way to charge them royalties now
4) the goodwill and perhaps recommendations from a satisfied customer, are worth more than whatever royalties you might be able to get out of them, because you probably wouldn't have any goodwill afterwards
You should check your contract to see what it says. In all likelihood, it is unreasonable to request royalties.
But in the future, it is a great idea to add into any new negotiations.
I have a client who loves something I made around 7 years ago as well, and he gives me a huge chunk of the revenue now because I've been helping him maintain it and modify it to fit new clients.
He's a good friend now. He said I do have a family and he didn't feel right paying me so little when others were charging him 20x the price or so. So he offered the fairer deal.
It's not right to change your rates years later. However, since you have the high ground here, use it to propose a new deal where you add new features for a cut of the profits.
Fair is a two sided affair.
Ask the client.
If the client feels it is fair, then it is fair.
If the client doesn't feel it's fair, it isn't fair.
B.
The cost of collecting royalties can easily exceed the value of the royalty.
Both in lawyers and accountants and in your time thinking about whether you are due and why you haven't been paid.
C.
If you want to make an ongoing business out of trade show systems, build and sell more systems.
You already have a happy client.
Talk to them about other players.
D.
Good luck.
You have someone who likes software you've written for them and is apparently good at doing business with trade shows. Probably there is other software which would be useful to participants at trade shows he could sell more of.
Just don't be especially surprised if his answer is that whatever he charges the trade shows covers an awful lot of manual work, setting things up, contact the other parties et cetera and the software is just a small but necessary part of that.
2. If you had asked for royalties in the start, you probably wouldn’t have been hired. Unless this was some problem only the top 0.1% of engineers could build, why would they hire you instead of some other person who wouldn’t ask for royalties?
3. That’s the nature of being a freelancer vs a business owner. You just built it and got guaranteed income. The owner still has to sell it and take risks to make it profitable
you should talk to a lawyer.
you can negotiate any deal you want but you will need a contract IMHO.