"Gold in our world is valuable because of its scarcity: all the gold ever mined in human history amounts to 165,000 tonnes, with perhaps another 100,000 tonnes still to be dug out of the earth. Middle Earth is (a) clearly much, much more liberally supplied with the metal, and (b) considerably less densely populated that our world. The unavoidable implication is that gold would be all but worthless in such a place."
From Adam Roberts’ review of Hobbit II (http://sibilantfricative.blogspot.co.uk/2013/12/the-hobbit-desolation-of-smaug-dir.html?m=1)
It occurs to me that there’s a certain logic here. If dragons and such like gold, despite having no use for it, it’s going to end up in their hoards. Any non-dragon gold represents either danger (if you’ve got enough, Smaug’s gonna come a-knockin’) or personal dangerousness (you’ve gone somewhere bowel-looseningly deadly and acquired it). In fact, both Moria and Erebor are evidence of how insanely dangerous gold-mining is.
In which case, the actual currency is not gold, but lethality. Gold is just a token of how appallingly dangerous you are. People with large amounts of gold command goods and services not because they hand over gold but because to have it at all they must be brain-meltingly dangerous, and you really want to do what they say.
Which, as I think about it, is perhaps less unlike our world than I had thought.
But look: this is how dangerous you have to be to get rich and stay that way in Middle Earth: Gandalf is broke.