I don't understand your point. If you're serving a file from AWS to ten people or to a massive userbase what happens ? Does AWS pricing change once you have a certain amount of traffic ? No, you pay per GB delivered. Apple should offer that kind of pricing, or a price per user (download) if they want.

And Apple should also be free to offer the % as an alternative if they wish, "don't want to bother with details ? Just stay entirely within Apple's system and pay 30% and that's it". But not strip the possibility of paying by use.

Because again, if I have 500 millions users, delivering a 5 MB update costs the same for a 5$ sub as it does for a 50$ sub, yet Apple's bills 10 times more to the second one.