It could be a little or a lot. 8 speeds aren't very common now so it depends in part on where the LBS orders the parts. They won't have them in stock. There are lots of inexpensive parst available or you could get "new" Shimano. Since the bike is old, a good LBS would go over the bike carefully and might include some adjustments. Anytime you change cassettes, you make need a derailler fine-tuning