I like having gold be relatively common compared to real life because it allows you to have large treasure hoards like the stereotypical dragon laying on a bed of gold without breaking the game.

In my campaign world gold is a little more common than silver is in real life and most gemstones are roughly on par with new world amethysts.

I also have "orichalcum" which is more valuable than real life gold and is used when I want a single hugely valuable piece of jewelry or high level players want to be able to carry their wealth around with them.