Since at max 100 ml liquids can only be taken through the security bottled water and similar have become quite expensive in duty free shops at airports.

Otherwise this an normal "kiosk" price. Less than in a pub or restaurant, but a lot more than in a normal supermarket.

For a 0,33 l bottle water/soft drink EUR 1,50 - 2,00 is the norm at kiosks. EUR 2,50 for 0,5 l does fit into this.

> 15c charge for the bottle

25 c deposit. By law. Same in a supermarket.

a) tap water is safe to drink in Germany

b) tap water is also more pure than anything what you can buy in a bottle

c) in some places it even tastes better.