There is a known 80/20 ratio: most profit is taken by a few companies. This makes sense as you would by "the app" not "an app" in any category.

Yes, absolutely, but it takes hard work and a good idea. Or rare luck.

I'd be more curious in dollars per developer-hour.

Numbers would get screwy throwing in every curious person who got skin-deep into UIKit enough to pop out a 3 hour TODO app.

There's a way to make significant money with free apps. You just need a decent idea.