Transaction fees from net or gross

The split between apple and dev is off the price shown in the store.

See the iTunes Connect pricing FAQ and your developer contract for details.

30% 'fee' is on the App Store listed price which is net of sales tax.

So e.g. when I offer an app that is € 9,90 for the customer (20% vat tax included in Austria), Apple takes € 2,97 (= 30% of 9,90). Is that correct?

Here's the short version: 30% is an approximation told to you because you're not an accountant.

There are enough accounting rules involved that you don't get 30%. What you get is determined according to the payment schedule in paid applications agreement, I believe still called Attachment 2.

VAT and sales tax rules are going to different between countries. In the US, you list an app for $0.99 (tier one) and the customer pays sales tax in addition to the $0.99. In a country where the tax is included in the displayed price, the payment schedule is usually -after- removing VAT.

But the answer to "How much do I receive for Tier X in Country Y?" is specified in the huge payment schedule chart.