Dwarven currency is based on precious and semi-precious metals. But the actual in-game intrinsic value of the coins are near zero because the only thing metal is good for is making into things, and when you melt the coins you'd only get a small portion of the metal back. Coins have some trade value, but trade will make up a very small portion of your fortress industry. Its main value is almost entirely relative, like any modern currency based on public trust, where a coin or bill has a RELATIVE value only because everybody agrees that it does.
But one of the central problems with the dwarf economy is that that it doesn't follow the fundamental principle of supply and demand, which is what sets the relative value of the currency; what the market is actually willing to pay based on how badly people want it versus how much is available. If you have nothing but legendary bedrooms, then the relative value of a legendary bedroom should logically plummet to meet the average income of your citizens. Because the supply of expensive housing is higher than the demand for expensive housing. If the prices were enforced, then the currency would devalue to the point that flipping a lever or harvesting a plant would be worth a handful of gold so that everybody could afford to have a bed.
Of course this is complicated by the fact that it's not really clear who is actually getting paid for use of the room, why they should be the one collecting, and what they can do with the money once they've got it.
This will doubtless improve in later versions.