B2C apps typically use a freemium model to make money off of in-app purchases and ad revenue. You could also make a paid monthly subscription model work in several categories, such as Newsstand. The challenge is finding the balance between giving enough functionality away for free so that your app can attract and retain users. Only if user retention is high does your app stand to monetize well over time.
A word of caution though related to free-to-play apps: http://m.ign.com/articles/2014/04/10/two-thirds-of-mobile-free-to-play-gamers-quit-after-24-hours.
Basically, the only safe bet in mobile development in particular (and business in general) is to create something beautiful, unique, and entertaining (or helpful), and then to market, market, market it and build a tribe of fans.
Hope this helps,