Ken makes several strong and pertinent points. The only thing(s) I will add relate to where you are in the review process and what manner of written info did you receive specific to the purchase and do you have any brochures with photographs that show finished ceilings? Unless you have a strong realtor on your side of the table it is likely that this part of the process will rely on the artful and aggressive maneuverings of your attorney. The bad news is that developer's contracts are geared to benefit the builder. Perhaps in the myriad small print of the contract this point exists and was overlooked.
Best of luck getting the ceiling done.
You've already agreed to buy the unit. If the contract is solid and all earnest money is in escrow, not much you can do but pay for the upgrade or leave it unfinished.
Upgrades are common and do vary but it's always imperative to know in writing when going into a contract what's included in a basic unit and what's an upgrade. This will prevent unwelcome and expensive surprises. A good agent with experience in New Construction would know the right questions to ask and make sure you knew that the ceiling was an upgrade at time of writing the contract.