If you are building a mobile app startup, you are probably looking to estimate how much your marketing cost will be. Here is a precise answer how to calculate it for any iOS app in any country's App Store / any category.
You can get the CPI of $1,5 and even $1,2 if you track sources of users and analyse their actions within the app. Then you filter out sources of traffic that sell you the dead users. You can do it manually (i.e. a small army of marketeers). You can also use automatic tools such as AppInTop.
If you look at the chart within the post, the last column shows daily revenue (in-app purchases). The advertisement revenue or offline revenue (e.g. selling of the posters made with those photo apps) are not included in the stats.
So, of course, Instagram does not have in-app purchases. And the Pic Stitch makes $824 per day (annualised to $300K). So the question is: is the app business about in-app monetisation, which works rather well for the gaming companies. Just use free AppInTop Spy service to look the revenue of gaming companies in the US (register free, press SPY icon on the top left hand side of the dashboard). Some make over $100,000 per day. But then the cost for the prime spots and the battle against King.com and Supercell costs much much more.