It seems like every time this story gets told the hoard gets bigger. Maybe movie people are dense and don't realize how much gold that would be. But having played D&D and read an article in Dragon Magazine about how many coins you can fit in a Portable Hole, I consider myself learned enough to weigh in.
The article, as I recall, placed each coin as about the size of a silver dollar, which is about what you see in the film. Loosely piled coins fit about 4 per cubic inch (each coin is about 1.5" across but very thin, and we're talking about large numbers of adjacent cubic inches here).
To know the size of the hall where Bilbo encountered Smaug, we need to know the size of Smaug. In a closeup we see that Bilbo's height is about 1/3 Smaug's head height. If Bilbo is about 3' tall, that means Smaug's head is 9' tall and 27' long. While we rarely see Smaug's whole body, it is more snakelike than lizardlike. In a human the head is 1/8th the height of the whole body, so for a dragon 1/12th to 1/16th seems appropriate. This gives him a minimum overall length of about 325'.
There was a shot where Smaug reared up and his walking wingspan was evidently only about 1/6 the width of the lair with his whole body spread out and coiled. We later see him slither and even fly through the room between the pillars.
We also see the hall with the gold statue being just about the width of Smaug's walking wingspan, and a comparison with how Bilbo fits into that room and the earlier lair room suggest the sizes line up that way.
Let's say the hall they fought in was perhaps 650' across and deep. For the coins to cover Smaug and still have a bed of them for him to lie on underneath, with Smaug's head about 9' tall, the coins would need to be perhaps 20' deep at minimum. Other parts of the room could be deeper, most were probably shallower, so let's give it an average depth of 10'. That's roughly 7.3 billion cubic inches, or about 29.2 billion troy ounces of gold that the dwarves had mined to date.
The Wiki estimates about 5.6 billion troy ounces of gold have been mined in the real world in all of human history.
That must be a pretty damn rich mountain.
We can assume that some of it was silver. But it would be a mistake to say the hoard was anything like 10% silver, because the dragon would have had to bury the silver at the bottom and cover it with gold - the hoard was obviously mostly gold from what we saw. And then some of it would be jewels, and objects crafted with skill that increased the value of the items beyond the gold weight. Using pure gold as an estimate seems generous given that much of the hoard could be richer and some poorer.
Gold is currently worth about 1200 per ounce. It's appropriate to compare to today's gold price and today's dollars if we want to make a comparison to what other things cost today. That puts his hoard at 35 trillion USD, or nearly half the GNP for all of modern Earth for one year, or 480 times as much wealth as the wealthiest person alive today.
All that assumes that Smaug's head is 1/12th his body length. If it's 1/16th, the hoard could be more like 47 trillion USD.
Here's a more reasonable depiction of the hoard. Still enormous, but not stupid.