The most expensive part of a sailboat is usually the hull and deck structure. Basically, the entire body of the boat. This is where top-quality materials and skilled craftsmanship make the biggest difference to safety, performance, and long-term value.
Other costly parts include the mast and rigging, the engine, and sometimes the sails, especially if they’re high-performance or custom-made. Interior fit-outs (like beautiful teak joinery) and modern navigation electronics can also add significant value.