Hollow-core doors are good for interior use only. They are lightweight, however they don't block sound as well as solid doors.
Hollow-core doors are much cheaper than solid doors. They can be as low as $30. When considering how many interior doors there are in a home, using hollow-core doors is a good way to cut costs.
Costs vary greatly by region (and even by zip code). To get free estimates from local contractors, please indicate yours.