30% is most certainly substantial.
Nope, not in retail.
This is why companies like Paypal or Square charge a fraction of that for financial transactions.
Has absolutely nothing to do with distribution, which is what we're talking about. Their percentage is in line with their industry [payment handling], around two percent.
Again, we're talking about distributors, though the App Stores fulfill multiple roles far beyond what distributors do in retail. Go talk to a record store owner, a bookstore owner, a coffeeshop owner, ask them what portion of their expenses go to their distributors.
Having had to deal with Apple's sometimes draconian rules around submission and rule conformity (see
here), I can certainly attest to the frustrations of having to deal with the rules.
I'm not disagreeing that Apple has strict rules and that some developers don't want to deal with those (see OnyX for an example of this). I'm saying that some people here don't appear to have much understanding of how retail works, what distributors do, and why very few companies have an issue with Apple's (Google's, Amazon's, MS) percentage.
Bottom line: if 30 percent was grossly out of line compared to what they could do DIY, why would ANY developer use these services? ANSWER: because it is in fact in line with or cheaper than going DIY. It costs money to bring a product to market. A lot of money.