This is quite difficult to answer because I am not very much aware of any successful nuke deal that has gone down in the black market, so we are basically gonna be writing based on research and speculations.
the black market is one of the biggest, if not even the biggest market in the world, it is also the only place where any kind of material or substance can be sold without being traced. (That if you really know your way around)
i have witnessed some huge black market deals go in previous times, and judging from those I would speculate that a Nuke would go for around $10 million at least.
One example we might all remember vividly was the time when the Al-Qaeda organization was shopping for Red Mercury on the black market. And at that time the price for black mercury on the black market was $1.8 million per kg.
Another incident was when a dealer on the black market actually tried selling cesium (a radioactive but not nuclear material) to the ISIS group for $2.5 million. Now cesium is usually used for making something called a “dirty bomb” which is capable of contaminating several city blocks.
At one time Uranium was been sold in the black market for a very cheap price at $360,000 per kg. Now just so you know, uranium is a substance used in making nuclear bombs, and 64kg of it was used to make “Little Boy Bomb.” The substance was said to have been obtained during the collapse of the Soviet Union.
Overall, scams seem to be the majority of cases. Even larger and relatively well-organized terrorist groups typically lack the expertise needed to determine if they are getting something legitimate.
But there is quite nothing to worry about (well, for now) because most of these cases were actually scams. even the terrorist groups mostly fall into the hands of scammers as they also do not have the expertise to differentiate or know if they are getting something legitimate.