It's not much cheaper too. Prices are from 30-35euro per day for small boats. But that is lowest price that you can get. At least, it was like that last year. In Croatia is not much more expensive, you can find berth from 40Euro.
Yes I have checked for a 41ft boat it will cost about 5000€ a year but the boat would have to be all the time in the water. They don't offer winter on land, summer on the water, they don't have the space for the land berths. That's about the same that most big marinas in Croatia, perhaps slightly less.