Here are some more information about the estimated budget for the trip...

Food :
- 15 to 30€ per day

Camping :
- 5 to 10€ per night

Ferries :
- 19€ from Rostock to Denmark
- 6€ Stubbekøbing to Bogø

Train back to Berlin :
- Between 25 and 50€

Other expenses :
- ?

Total :
Between 200 and 400€ per person for 8 days

Ok, this is a really rough price estimate..
I know exactly how much it costs for one person but for 5 or 10, it's harder to estimate.

The idea is that we'll stop in supermarkets to buy stuff to make sandwiches for lunch and pasta/rice for dinners.
So food-wise, it should be relatively cheap. And then camping should be cheap as well.

I'll probably pay for the expenses during the trip and we'll split it at the end to make it easier.

Edit (after trip) : Final price is 120€ per person for the whole trip (excluding the ferry). Reasonable price I'd say.