See here for my estimates on the value of a gold dragon.
As for whether it’s a realistic pot, it’s a bit tricky, because medieval tourneys don’t seem to have given out prizes in cash, but instead gave out prizes in jewelry, plate (hence why so many modern sports tournaments have “cups” as trophies), and the like. And without these objects to hand, it’s a bit hard to value how much a “gold vulture” or a “very rich ring” should be valued at, or (given how popular diamonds were in medieval tourneys) how to appraise precious stones in the abstract.
However, I can say that 10,000 dragons work out to something on the order of 6,000-7,000 English pounds (in 1300 CE pounds, that is), which is far, far bigger than any tourney prize I can find an example of.
So I think this another example of math being GRRM’s Achilles’ heel.
Yeah, and Sandor’s 40,000 gold dragon prize for the tourney is insane. (24,000-28,000 English pounds? What in the world could you buy with that? And Anguy spent his all on fancy food and fancy girls? How?)
In comparison, Jaime Lannister believed his own ransom value was 10,000 dragons… yes, the value of Jaime to his father and to the crown was the same amount as the first prize of an archery tourney, the hell GRRM. (Or the hell, Robert.)
And when you compare that to D&E’s value of a fine palfrey stated to be about 7 dragons (twice as much as 3 dragons plus a bunch of silver)… well, either there was a lot of inflation in 100 years, or GRRM really can’t math.
Heck, the fact that 9000 gold pieces, the amount Sandor had on him a year later when he was captured by the Brotherhood, weighs about 180 pounds (80kg or so), makes me really respect the carrying capability of Stranger the warhorse…
Yeah…with that kind of money, you’re either talking about buying large chunks of real estate, multiple ships, or buying your way into a fiefdom (by way of dower, I suppose).
As for Anguy, I imagine that part of it is him living the highest of lives, buying “rounds” on the house for everyone else at Chataya’s, and being robbed blind because he’s a country boy who doesn’t know how much fancy things cost.