We know from the novel [The Hobbit] that Smaug’s wealth comes down to three primary components, the mound of silver and gold that he sleeps on, the diamonds and other precious gemstones encrusted in his underbelly, and the “Arkenstone of Thrain,” which is depicted as something like the Hope Diamond on steroids. (There are certainly other valuable items in Smaug’s hoard – rare suits of armor and so on – but the point of the exercise is to establish a minimum, conservative, net worth and the total value of a pile of ancient weaponry is probably no more than a rounding error in a fortune measured in the billions of dollars.)
Using a bunch of educated estimations and math that I don’t care to understand, Forbes’ Michael Noer estimated that Smaug’s fortune is worth approx $8.6 billion.
That brings him up to position #7 on Forbes’ list of the 15 riches fictional characters. Scrooge McDuck is #1, naturally, with a total value of $44.1 billion.