They are pretty expensive, I can't remember exactly how much though.
I think Santa Eulalia is cheaper than Ibiza Town.
If you want it year around, it is best to join the Club Nautica in Santo Eulalia as they have some discounted berths for members.
You can only join at a certain time each year, I seem to remember it being in June, and they only have a number of places.
The Bouys tend to be privately owned and the policy depends on the beach.
Some beaches are governed by the beach bars who manage to charge for the privelage.
Other beaches, you can get away with it as long as you are discreet. We have a rib moored most of the summer for free.