Sure, some people can. The app obviously needs to be good and fulfill a need. This isn't as easy as it sounds.

What are your theories on how to make money with apps?

Apps are the same as any other business; you need a good product that people want, and you need good distribution. Most developers will fail when it comes to distribution. After that, most apps will be free and offer in-app purchases (unlock or buy items), or use ads to generate revenue.