Commodities that are consumed are priced on the marginal cost of production. Saudi Arabia can pump oil for less than $5/barrel, but what drives the prices are the costs of fracked shale and frontier plays like UDW. Gold is the same. Generally, gold is found in sulfide mineralization deposits or in placer deposits (weathered ore bodies, ie nuggets in alluvial deposits). Most new mines are the former, which requires massive mining and ore treatment (on the order of grams of gold per ton of ore, which is then crushed and leached in acids). So it's expensive to produce. Usually it's found with silver and copper, and some miners will account for it after credits for selling those metals, but you can assume $800-1000 USD/oz for production costs after fully capitalizing the development of the mines, at least the last time I looked into it several years ago.

There's a lot of assumptions built in there... but it's besides the point: n% is infinitely better than 0% for any value of n > 0.

Also, the claim that "gold only became a currency when it was stamped" is wrong. The value of gold on the commodity markets today does include a value store component and none of it is stamped.

When the czar's family fled russia they didn't grab rubles, the grabbed jewels. Why? Because their currency was worth nothing but their jewels could be traded for value in a very transparent manner wherever they might end up.