Slovakian police arrested 2 men who possessed about 500 grams of uranium that they intended to sell (ABC has the best online report I've seen so far).
This was enriched uranium - the kind that governments have to spend big buck to learn to make for a bomb (and the kind that Iran hasn't figured out how to make yet).
And yet, it wasn't enough for a bomb - it was about 2% of what you need for a bomb.
And ... it was powdered. You need a lump for a bomb.
So, this is exactly what you would need to make a radiological bomb: one in which conventional explosives spew radioactive material into the air.
That much uranium wouldn't be too harmful - but in a world where people call microwaving "nuking" and are afraid to eat irradiated foods - the panic associated with its release would be huge.
So, here's the economics. The going price for this stuff is $3500/gram (about 30 times the street price of cocaine). These guys had just under $2 million worth.
This stuff doesn't cost $3,500/gram because they picked that number out of thin air. It costs that much because there is someone on the other end willing to pay that much.
Now, take off your foil cap and put on your thinking cap: if no one out there wanted to use this it's price would be $0/gram. That's the going rate for useless stuff, like say the used post-it notes around your desk.
In fact, if you had 500 grams of enriched uranium, the value to you would probably still be zero because you wouldn't know who to sell it to. The only reason for having it is because you are privileged to know who to sell it to, and how much they'll pay for it.
This economist's conclusion is that if you don't think a terrorist is going to build and use a radiological bomb, then you are in denial. The counterfactual is that if you are right, the price for the stuff would be a lot lower than it is.