If you're looking for a reasonably priced ballroom, don't forget the military. Local military bases often have beautiful ballrooms and banquet halls. Some may only rent to veterans or active military personnel, but others are willing to rent to the public.
Remember: no matter how nice it is, a ballroom is nothing without music. At the very least, you'll need to hire the services of a disc jockey, or ideally a complete band. The type of music depends on the type of dancing, of course, but it's a good idea to get a band or DJ with some flexibility in their style.