Not making a living, but I make about $200/mo from an API that parses recipe ingredients like "2 cups finely chopped onions" into structured JSON.[0]
I put it in maintenance mode in 2019, so it's about 99% passive income, as I spend only about an hour or two on maintenance per year.
I'm surprised all my clients haven't switched to LLMs, but maybe I still outdo LLMs on price/accuracy since it's so niche.
I'd like to sell it to someone who wants to do something with it, but it would probably take me 30-40 hours to package everything up to hand off to someone, so I consider just the opportunity cost there to be around $5-10k, and I don't think anyone wants to pay $10k for an API that makes $200/mo.
What I wish I knew: don't use RapidAPI. They charge 20%, they have a terrible interface, and they let customers run up huge charges and walk away without paying anything. I wish I'd just rolled my own simple thing with Paddle.
[0] https://zestfuldata.com/