Many ballrooms require that you put down a substantial deposit when you book the venue, often amounting to hundreds of dollars. You can generally consider this to be your cancellation fee if you pull out, but do check your rental agreement before making this assumption.
If you're looking for a good place to hold your wedding reception, it's a better idea to book a ballroom than a banquet hall or similar venue. Ballrooms are ready-made for dancing, and most come complete with the tables and seating you need to be comfortable.