I want all of my dragon-in-her-lair fights to be the Hobbit’s Smaug scene or Aladdin’s Jafar fight. I want the terrain to be coins, so many coins.
There’s this size-Gargantuan 20×20 dragon, right? And so an interesting fight against it happens in a lair with enough room to move around. If it were a 5×5 orc, you’d want a 20×20 room for the fight to stretch out in — 4 times bigger along each dimension, so let’s say an 80×80 lair. That’s 6400 square feet. That seems appropriately cavernous.
Some clever soul says a silver coin is about an inch across. So a lattice one layer deep and one square foot across is 144 coins — let’s call it 150 coins.
So a layer one-coin-deep in this dragon’s lair is 960,000 coins. That’s about on track for the value of the legendary hoards we expect this dragon to have, since it gets some multiple number of hoards per the rules. But it’s one coin deep. Isn’t that unsatisfying? I want it to be hundreds of times greater than that; great piles of wealth.
Oh, sure: the dragon could be sleeping on pennies. You know that’s not good enough; it’s gotta have the glint of gold. So our only real option is to cheat; to say that the pile is one hundred times, one thousand times deeper than that single-coin stack, but that the heroes don’t get to keep it.
Perhaps there’s a tax on dragon gold in these parts. But really, a 99% tax? A 99.99% tax? That induces most PCs into outlawry.
Perhaps dragon-gold really is toxic to mind or to spirit; keeping or spending that wealth renders one rapacious, sickened, or cursed. I think that’s the least gameable answer, though, because the PCs killed a dragon; of course they want the hoard. And they have expert saints and wizards and suchlike in the party, so they can reverse most curses. And they’re played by smart folks, engineers and economists and such; even if the curse is unbreakable, they’ll figure out some system of catspaws to defeat that. So it’s definitely an option, but not the best.
Perhaps dragon-gold is mostly shadow-stuff — the hoard does contain coins, but they evaporate once away from the dragon for an hour or a day. You can still get some money out, but at dissatisfying one-in-a-thousand coin rate.
Perhaps dragons destructive powers wreck their hoards, melting and corroding coins. This has the downside that if clever players can kill the dragon away from the baubles, they get a vastly oversized payout.
How vast is that payout? Let’s say our dragon had a conservative 100 million gp in her hoard. Then that’s 2 million pounds of gp — 1% of the world’s gold supply. The current world’s gold supply. A more realistic medieval estimate puts that figure at 300% of the then-world’s gold supply.
So: it’s gotta be “Elemental Plane of Gold”-level fantastical.