So the Apple store 30% tax would be more like a 30% VAT on the value created by the app - excluding the value already present in prewritten libraries etc the ap

So the Apple store 30% tax would be more like a 30% VAT on the value created by the app - excluding the value already present in prewritten libraries etc the app builds on. The problem there is that it's impossible to quantify that. VAT works in-practice in retail sales because the value added is trivial to calculate - but you can't do that with open-source libraries or internal components - or the value created by a platform (e.g. what value does Spotify add on-top of the existing iOS audio and internet capabilities that they're using?)