One developer had a free app to track some child health data. It was long time ago so I don't remember the exact data being collected. But when asked about the economics of his free app, the developer felt confident about a big pay day.
As per him the app's worth was in the data being collected. I don't know what happened to the app but it seemed that app developers know what they are doing when they invade privacy of their users - under the guise of "free" app. After that I became very conscious about disabling as many permissions as possible and especially not using apps to store any personal data, especially health data.