I don't think that money has a value that can be reproduced over time any more. Notes promise to pay the bearer £1, without saying what £1 is.

As has been said, the price of currency is what people are on average prepared to pay for it. The dollar is to a large extent propped up by Eastern countries buying back their own trade surplus as they want to continue to sell goods. How many billion is South Korea, China and Japan sitting on now? I loose count.

Gold was OK when countries aren't linked, but what happens when a market gets flooded with gold (or silver as was also used as the base precious metal). Americans used "trade dollars" made of silver and worth $1, but not redeemable in the USA itself to trade abroad. Every time these systems were abandoned was that the fluctuations of its value were too great. On a large scale the website fails in the same way.