Are core buildup and new pins necessary if the dentist only needs to replace my existing crown?

If your dental crown is chipped and you need to redo another crown, the crown usually needs to be cut out! Then at that time, the dentist will evaluate if tooth build up or crown build up is needed! So AFTER the crown is cut off the decision will be made, rarely prior to removal of crown. Since the xrays do not let the dentist see if there is enough tooth structure is remained prior to placing a dental crown.
In Instances in which there is no crown on the tooth, but the tooth requires a porcelain crown restoration, it is much easier to know if tooth build up or post is needed PRIOR to placing a crown on the tooth. Hope that clarifies it for you.
Best of luck

It is a judgement call on the part of your dentist. If upon removal of your chipped crown he feels that there is inadequate support for a new crown the buildup would be indicated. However, I'm not sure how your dentist would make that call before removing the chipped crown

Your quote includes a build up for a tooth under the old crown due to a possible scenario of such build up being needed as a result of many issues your dentist may find under the old chipped crown. Such issues may include tooth partial fracture while attempting removal of the older crown, removal of recurrent decay under the old crown and resulting structural damage to give you a few. Your dentist may find during the procedure that core build up was not necessary after all and not charge you for that. Best way to address this question is to ask your dentist and share your concerns. Good luck, Dr. Daniyar

Replacing crowns often requires replacement of the buildup (restoration under the crown). Sometimes when crowns are removed, the buildup can become loose, or it may have decay underneath it. It is your dentists responsibility to insure your crown is placed on a healthy tooth structure, therefore a new buildup is recommended if required.
Hope this helps,
Dr.Mazaheri