My friend and I built https://apitude.co/ around 4 years ago as a side project. It consists of a group of REST APIs like this: https://apitude.co/en/docs/services/renapo-mx/

The site did: - $200 a month the first year. - $900 a month the second year. - $2K a month the third year. - $5K a month the third year. (We started working full time). - $9K a month this year.

Even when it was only a side project it demanded a lot of work, but we love the space so it was ok.