Abby made over $110,000 from sales of digital products (such as eBooks) in 2015, and even more than that in 2016. She started three years ago knowing nothing about blogging and now makes six figures a year, due in large part to her eBooks. She is now teaching others the process she uses to write and launch profitable eBooks, and you guys, it is genius!
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.