Sure, there are rich people with agendas, but they typically want to keep their money and power. Nuking people tends to go in the opposite direction for them, both financially and health-wise.
And the difference between me and you on this is that I would never leave this up to chance.
What would you do about it? The only two options I see are
1) Give the power to control (and thus own) nukes only to a select few, whom we'll call "government," and hope that none of their members become unstable and try to set one off
2) Make it an acceptable community standard that no one in that community should own nukes, and make everyone be responsible for keeping an eye on each other, and report/stop anyone from trying to obtain weapons-grade nuclear materials
Personally, I think option 2 will be more effective.
I'm curious, though, why do you hold government in such high regard exactly? What makes a person who works for the government different from just any other person? And if they are the same - just people - then how will government employment prevent someone from becoming unstable?
Just take that example, A. you would set it off in your backyard (hopefully) B. maybe you investments are in oil, setting that thing off would make the price go through the roof..... bam, profits. Hell we even have major short position before the WTC disaster on the airlines that were historically way outside the normal size of the shorts, who ever held those made a ton of money. Please, widen your horizon.
Anyone setting off a nuke will very likely be found out. A nuke isn't just something you can build in secret. So if you were to blow up a nuke in your back yard, not only will you be losing your back yard and all your neighbors, but you will likely make sure that no one will ever buy oil from you again, and will end up with a HUGE price on your head.
Otherwise, how do you propose that whoever does that in your example manages to get away with it?
Because of the destructive nature of nuclear weapons, obviously I would rather no one have them. But in reality, the cat is already in the bag and because competing nation states have nuclear weapons in their possession, they would likely never relinquish them all because of the fear the another nation state was lying and held some back in their possession. As for unstable people getting access that are in government, you should research the current safeguard we have in place so that only a few men have the ability to arm and launch nuclear weapons and those people are vetting quite harshly and are given continual physiological evaluations you mental stability.
That is an assumption they would be found out, if you know what your doing you could be covert about it. There are small tactical nukes are are as small as a suitcase. Also in your example, you gave a price tag so I had to assume that person was purchasing it from somewhere not building their own. Building your own it a whole other ball game and is very hard to mask because of the equipment needed, power consumption and raw materials. You really need to understand, covert action happens everyday and it is effective more times than not. History is filled with examples (that we know of) and it is only decades if not longer that we find out, or we seize records and read about them after the fact.