I can only speak for my own calculation model, but that is simple:
If I need 2 hours, I invoice 2 hours
If your design- and functionality wishes/ideas/needs are so special, that i would need 8 hours to create, I would invoice 8 hours.
The usual price/hour depends on the market, so if a big brand is using a sophisticated design exclusively, it can rise up to 200.-€.
But on the other end, for low budget offerings, it can start from 40.-€
(all prices: plus tax, if applicable).
Take in mind, that there may also be some extra hour(s) for:
- communication and explanation time
- data transfer handling time
- correction loops
If you want to see some of my works and designs: