~This is a really bad idea in my opinion because the total cost of operating the App Store would be equally distributed amongst all apps, which hurts small developers and benefits big corporations disproportionately. An app like Facebook, which is downloaded billions of times, does cost Apple significantly more than a small indie app used by, say, thousands. You can't get around that. Having the small indie app paying for Facebook (indirectly) is a lot worse than the current system. Worse, Apple is unlikely to decrease its profit margin. This would mean small developers and big corps pay an equal share in the profits Apple is going to make.~

~A better idea would be to have a free tier and take a percentage above that. The first $1000 are a lot more valuable to the creator than everything beyond that.~

edit: OP said per download, misread it. That makes sense.