Refining potentially fissionable material is difficult. Remember that U-235 and U-238 are chemically identical, so you can't separate them by any chemical mean

Refining potentially fissionable material is difficult. Remember that U-235 and U-238 are chemically identical, so you can't separate them by any chemical means, and U-235 makes up only 0.7% of natural uranium ore. The Manhattan Project employed 130,000 people... many of those were NOT the guys designing the bomb, but the guys trying to set up refining operations and the like. It cost the equivalent of $21 billion dollars in today's money, and took five years to get a bomb. So, if you have a country that has $21 billion dollars and five years to spare, and can avoid getting bombed by the United States during that time, and is sitting on minable fissionable materials, they can develop a bomb... Even if you give them a 50% discount because it has already been done, still, $10 billion dollars.