This provides no basis for comparison because the App Store has been the only way to install apps on iOS that whole time, so that changing can't have been the difference.

Moreover, the market they're monopolizing isn't the market for apps, it's the market for app distribution. So the relevant price isn't the price of apps, it's Apple's 30%. Which is obviously higher than in markets where there isn't a single-party distribution monopoly, e.g. payment processing is typically ~3%, various packaging systems like apt are free (or user can download binary from website as on Windows), and the binary hosting cost per user is negligible.

There is also a tying argument. App Store ties payment processing, distribution and curation together when the buyer might want different providers for each.