How is a testicle worth on the black market?

There is no evidence that there is any kind of market for testicles.

This question likely refers to a 1994 edition of the "New England Journal of Medicine" that published an article claiming that some testicles were being sold for a high price on the black market so they could be used to rejuvenate aging men. However, the article turned out to be satire. There does not appear to be a market for testicles of any kind.