There are universal apps, and there are also separate apps for the iPhone and iOS. While different businesses will take different approaches, it’s always important to plan for the future when pricing your app. Questions such as “why should screen size determine an app’s value” should be asked.
The clearest example of this concept is OmniFocus, a productivity app that costs $20 on the iPhone, $40 on the iPad, and $80 on the Mac. This pricing policy makes sense at first glance. The increased display sizes of the iPad and the Mac could allow for more features than the comparatively cramped display on the iPhone — since it makes little sense to charge the same amount for a less capable product, the current system appears to be the best choice.
But the iPhone app isn’t less capable than its larger counterparts. It better supports location-based features, was the first to be updated for iOS 7, and is probably used more often than the other apps. (It’s easier to use a task list on a device in your pocket than it is to use one on the computer sitting on your desk at home.) And the iPad app is often considered the best version of the app because of its ease of use and, previously, access to exclusive features. Saying that these apps should have different prices has little to do with the apps’ functionality and everything to do with App Store economics.