Using an ASO-only model (=App Store Optimization) with no marketing costs whatsoever, the app was bought almost 500 times in two years which results in a profit of around $1k. It's not much, but since the experiment only took two weeks to develop, I count it as a success. As the app doesn't use a backend and there's no need for customer support, this is the closest I've come to generating real (minimal) passive income so far.