Probably something that is so scarce in earth that the monolith alone couldn't have been made by anyone other than a government, a billionare or aliens. Solid gold maybe?
On a more serious note, Rhodium fits the bill: super expensive, hard, durable, silvery and corrosion resistant.
Using the 7,700 lbs estimate elsewhere here, and a price of Rhodium I randomly found on the internet of $16,100 per troy oz, the material of this structure, if made from Rhodium, would cost about $1,749,005. Which is actually not bad, for a very rich person. I guess they'd have to build an even larger one. Or make it hollow and fill it with printer ink.
I think you calculated that wrong. I get $1.8B at that price.[0] Though that 7700 lb estimate assumed the sculpture was made of aluminum; rhodium is about 4.5X as dense, so it'd be closer to $8.1B.
Ah, thanks for the correction. I just quickly googled values and multiplied them but thinking about it, I probably multiplied by "lb/troy oz" instead of "lb/troy oz" or something like that. I'm an idiot :)
Your figure does sound a bit more realistic. Still, printer ink...
