Earlier quoted context omitted.
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?
hah. gold is not that rare. even an entry level billionaire could afford a monolith this size
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.