Funding a minimalist nuclear weapons program is within the grasp of many billionaires. It's also well within the grasp of all but the very poorest/smallest countries; North Korea managed just fine.
South Africa's successful but later dismantled nuclear weapons program cost about $400 million (1993 dollars), which is about $865 million (CPI inflation adjusted) today [1]. Cue the discussions about whether the cost of nuclear weapons production has been changing more in line with other manufactured goods like TVs or more like services such as health care.
The main obstacle for countries, or villainous billionaires, is how to build nuclear weapons without being detected. That's a lot harder, particularly if you want a live fire test to verify that the design works. (A basic gun-type bomb like the US dropped on Hiroshima, or like South Africa made, is so very simple that it can be assumed to work without prior testing. But it's also an incredibly inefficient way to make a bomb compared to implosion designs, which need live testing.)
South Africa's successful but later dismantled nuclear weapons program cost about $400 million (1993 dollars), which is about $865 million (CPI inflation adjusted) today [1]. Cue the discussions about whether the cost of nuclear weapons production has been changing more in line with other manufactured goods like TVs or more like services such as health care.
The main obstacle for countries, or villainous billionaires, is how to build nuclear weapons without being detected. That's a lot harder, particularly if you want a live fire test to verify that the design works. (A basic gun-type bomb like the US dropped on Hiroshima, or like South Africa made, is so very simple that it can be assumed to work without prior testing. But it's also an incredibly inefficient way to make a bomb compared to implosion designs, which need live testing.)
[1] "A Chronology of South Africa's Nuclear Program" https://www.nonproliferation.org/wp-content/uploads/npr/masi...