I can only assume the same way most rare items are priced - whatever the buyer and seller negotiate. The other option might be auction, but this doesn't exactly work for nuclear materials.
If the seller produced it, then they would presumably charge a hefty premium over the cost of production. If it "fell off the back of a truck", the seller would probably sell it for a huge discount to the real value because, after all, there won't be that very many buyers, and what buyers the seller can find might not be able to muster the real value (or at least they'll try hard to pay much less).
It says you need nuclear reactor and particle accelerator. I guess entry to barrier is sort of high.