Before open source was a thing, I used to sell my code as shareware. The compiled library was free to use, but had some limitations. The source code version cost money and was not limited. I originally wrote the code to scratch my own itch, but it turned out to be useful to others.

I made a bit of money from it, but ultimately the support ate up all my time, and it was not worth it in the end.

These days I just open source my code. I still love writing it and I get great pleasure to see others getting value from it. I earn enough in my day job that I don't want to turn the things I do for fun into another job.