There is also the buying mentality that an app store user has when browsing for apps. It's hard to measure this but is probably bigger than anything else you get for your 30%. You sell an app store app and people don't mind droping 99 cents or a couple of dollars for something that preforms a few functions.

In contrast I think web apps are a much harder sell at those price points, so for a similar app your only real choice is advertising supported.