black-market price
Refers to the price of goods sold illegally or outside official channels, often at inflated rates.
戦時中は米が闇値で取引されていた。
During the war, rice was traded at black-market prices.
この薬は品薄で、
This medicine is in short supply and has a black-market price.
A compound of 闇, in its figurative sense “illegal or black-market,” and 値, “price.”