In a way, it’s a trend that should have been foreseen. A look at the App Store or the Play Store shows that free apps are making more money than their paid counterparts. And they aren’t really free in the actual sense of the word. From ads to subscriptions, to in-app purchases, there are a variety of ways these apps can be monetized. The days of the 99 cents apps are long behind, freemium and the like have taken over now.
There is no definitive way of knowing what will work to monetize your app. If for whatever reason, you’re absolutely convinced that your app is something people can’t live without and that it enriches their lives, you can charge probably two or three dollars. But seeing as how people want things for free, this might not be the best route. A better option would be keeping in mind ads and in-app purchases from the get-go.