Michael Noer of Forbes, compiler of the Fictional 15*, nerds out and shows us his process for estimating the value of Smaug's treasure hoard. Discounting nigh-unvaluable items like mithril shirts and assorted antique weapons (but including the Arkenstone of Thrain), he sets out to estimate the dragon's size--using tables from Dungeons & Dragons--and determine a minimum value based on the minimum size of a pile of gold and silver coins large enough for it to curl up on. Then he adds the minimum area of diamonds necessary to cover Smaug's belly (except, of course, for the large patch in the hollow of his left breast), uses an ingenious formula to value the Arkenstone, and ends up at 8.6 billion dollars, seventh on the list. Beautiful.
Then an even bigger nerd points out that his size estimate for Smaug is unreasonably based on a misinterpretation of the D&D table. Correcting the figure, and using textual evidence from The Hobbit for support, he reruns the math and gets a minimum of 191.7 billion bucks, excluding the mithril shirt whose value Gandalf says was "greater than the value of the whole Shire and everything in it".
Suck on that, Scrooge McDuck.
[* - Which is awesome, if flawed; Bruce Wayne is worth many, many times more than Tony Stark.]