Coins are Big and Heavy
"How many boards could the Mongols hoard if the Mongol hordes got bored?"
From the standpoint of the adventurer, the primary difficulty of the D&D currency system is that the lack of a coherent banking and paper currency system means that there are profound limits to what you could possibly purchase even with platinum. But the currency system hurts on the other end as well. Untrained labor gets a silverpiece a week. That's 500 copper coins a year, which means that no matter how cheap things are they can only make one purchase a day most of the time. That's pretty stifling to the economy, in that however much gets produced, noone can buy it. Demand, from the economics standpoint, is strangled to the point where large production outputs don't even matter (remember that in economics Demand doesn't mean "what people want", it means "what people are willing and able to pay for", so if the average person only has 500 discreet pieces of currency per year, that puts an absolute cap on economic demand, even though the people are of course both needy and greedy enough to want anything you happen to produce).
What's worse, those coins are heavy. For our next demonstration, reach into your change drawer and fish out nine pennies. That's a decent lump in your pocket, neh? That's about one copper piece. Gold pieces are smaller (less than half the size, actually), but weigh the same. D&D currency, therefore, is more like a Monopoly playing piece than it is like a modern or ancient coin. There's no reason to even believe these things are round, people are seriously marching around gold hats and silver dogs as the basic medium of exchange.
Now, you may ask yourself why these coins are so titanic compared to real coins. The answer is because having piles of coins is awesome. Dragons are supposed to sleep on that stuff, and that requires big piles of coins. Consider my own mattress, which is a "twin-size" (pretty reasonable for a single medium-size creature) and nearly .2 cubic meters. If it was made out of gold, it would be about 3.9 tonnes. That's about eighty-six hundred pounds, and even with the ginormous coins in D&D, that's four hundred and thirty thousand gold pieces. In previous editions, that sort of thing was simply accepted and very powerful dragons really did have the millions of gold pieces – which was actually fine. Since third edition, they've been trying to make gold actually equal character power, and the result has been that dragon hoards are… really small. None of this "We need to get a wagon team to haul it all away", no. In 3rd edition, hoard sizes have become manageable, even ridiculously tiny. When a 6th level party defeats a powerful and wealthy monster, they can expect to find… nearly a liter of gold. That is, the treasure "hoard" of that evil dragon you defeated will actually fit into an Evian bottle.
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