That will depend on the finishes you choose and the qualify of workmanship you are willing to pay for. Try asking a few local contractors if they could give you an estimate. You will learn a lot by listening to them. If you are not familiar with prices and materials, try spending a few hours in a home store like Lowe's and flooring stores, building supply stores, stone companies, and appliance stores. That way you'll have a better idea of what the finishes look like and what retail costs are for the various products.
I would be happy to offer you a list of contractors. Most contractors will be able to show you projects that have been completed, offer references and price quote. Please feel free to email me at firstname.lastname@example.org
Bucky - try these guys. They built a beautiful home for my parents a while back that we just sold, but were fantastic to work with. I think Louis' answer was on the money (no pun intended). But they'll be able to give you an accurate, on-island answer. Please tell them I sent you.
Bucky, Nantucket offers its own challenges. It will depend greatly if you are using local talent or bringing someone over from the mainland. Many contractors commute due to the lack of supply on the island. If you would like to speakk to a trusted contractor, let me know.
You can do a bathroom from 5,000 to 100,000 and kitchens depending on your choice of cabinets and appliances can be from 20,000 to 150000. Just built a home in Newton above average appliances 20,000 custom cabinets, 40,000 granite 6,000