Is it? How do you break it down to being a 'good deal'?
Serious question though, how can it be shown that 30% is not 20% too much and Apple is still making a serious profit from it. The issue right or wrong is that when there is only one option, App Store, Play Store, Windows Store or whatever on each platform, scrutiny follows as to how much money is too much for big companies.
I honestly don't know the answer, neither do I really care too much. When I need it the App Store is fine.
Although there is a lot of complaining about the 30%, I wonder if they reduced it all the way down to 5%, would the developers pass that saving to me? Probably not, so I agree purely on a 'my opinion' basis that 70% of the sale for not having to handle all the downloads, payments, refunds, fraud and so on, seems fair. That is a lot of work developers don't need to worry about.