The transuranium elements, in general, are extremely expensive. These elements typically are man-made, plus it is costly to isolate the trace amounts of transuranic elements that exist naturally. For example, based on the cost of the accelerator time, man power, materials, etc., californium is estimated to cost around 2.7 billion dollars per 100 grams. You can contrast that price with that cost of plutonium, which runs between $5,000 and #13,000 per 100 grams, depending on purity.
Antimatter Costs More Than MatterOf course, you could argue anti-elements, which technically are pure elements, are the more expensive than regular elements. Gerald Smith estimated positron could be produced for about $25 billion per gram, in 2006. NASA gave a figure of $62.5 trillion per gram of antihydrogen, in 1999.
More Element FactsWhat Is an Element?
How Are Elements Named?
What Are the Elements in the Human Body?
What Letter Is not on the Periodic Table?