Osmium
The cost of osmium as of January 2010 was $12,217 per kilogram, making it roughly twice the price of rhenium, which is worth $6,250/kg. Rhenium has two naturally occurring isotopes, 185Re and 187Re. Irradiation by slow neutrons would transmute these isotopes into 186Re and 188Re, which have half-lives of 3 days and 17 hours, respectively. The predominant decay pathway for both of these isotopes is beta-minus decay into 186Os and 188Os.
Read more about this topic: Synthesis Of Precious Metals