Because it can only be found on a deceased person in a coffin--and indeed is said only to grow on the skulls of persons who consumed high-quality ginger for long periods of time while alive--supply is naturally very limited, and the price is correspondingly outrageous.
由於是棺材中死者身上的東西，而且據說死者必須在生前長期服用高貴人參，才會生長出來，貨源相當稀少，價格自然昂貴。