A percentage doesn't make sense, because cheap apps and expensive apps require exactly the same effort for hosting them, and there is no risk involved.
So the fact that these competitors use the same pricing model and the same price has a bad smell.
If the actual cost of service was pro rated across apps this would basically eliminate free and low cost apps. In fact how would you price this in the first few years of the App Store when it lost money? Furthermore it would make it incredibly expensive and therefore risky to develop launch an app. They would likely have had to charge hundreds of dollars, perhaps thousands per app in the early years, stifling innovation and strangling freemium or free offerings. The App Store would be a very, very different place today.
This would be like supermarkets charging product manufacturers a uniform fixed rental for shelf space. No supermarket would stock toilet paper, it’s too bulky and low revenue, or if they did they have to charge a lot more for it.
The 30% commission has a long history. There are precedents going back to the early days of games consoles. Both the iTunes music store and Steam opened in 2003 charging 30% or very close to it (iTunes tracks were 99c so it doesn’t work out exactly).
It’s hard to argue the 30% in either of those cases were a matter of abuse of market power. Steam was struggling to establish itself against the incumbent distribution channels. The iTunes rates were negotiated with the major labels who were notoriously hard deal makers and very wary of online sales channels. They held all the cards is their negotiations with Apple and yet they seemed to think 30% was fair. So at that point I don’t think it’s possible to make a credible case that the 30% rate was extortionate.
If 30% was abusive, we’d expect to see it act as a brake on adoption of the App Store by developers, but is there really any evidence for that? On Android have any of the smaller stores tried to differentiate on price at lower than 30% to attract developers, or has Google tried to woo iOS developers with lower rates? If 30% was abusively high we’d expect to see something like that happening.