This is looking more and more like a smuggling operation gone bad all the time...
According to some estimations , cost of Polonium-210 used to kill Litvinenko is around £20 million ($ 39 million), which would make it the most (?) expensive murder in history. However, this estimation seems to be based on retail prices of commercially-availible demonstration radiation sources (such as those sold by United Nuclear, for example), which contain very little quantities of Po-210 (one would need about 50 000 pieces to get the dose used in this poisoning). For more plausible, and much lower estimations, see above: the cost could be as low as only $ 7000.