I think the decorator will charge for the amount of work they put into it. My mom had a decorator come in and just make suggestion and do a walk through and it was inexpensive. It's also regional and depends on how experienced and popular the decorator is. They can do everything, you give them an empty, white room and they can turn it into whatever you want. Call a few decorators and ask for a consultation at their office. Bring pics of the room you want decorated, it'll probably be free for the initial appt.
If you decide not to go with a decorator you can always find a pic of a room you like and try to duplicate it. Have you been to the Land of Nod's website? They have amazing rooms.