The Pauper prank was good. You may be right about Galleons being made out of gold. There are two problems with your reasoning. One, gold (and silver) have in many cultures been used figuratively not of the literal metal but of money in general. That may be what wizards are doing by the references to gold. Secondly, monetary value and metals do not always match (it's one of the reasons why the U.S. made steel pennies during WWII and why they will either do away with the penny or start making them out of steel or something like that; copper pennies cost more than a penny to make. Further, JKR is rarely consistent in her tales -- I would not take it seriously that galleons are to be reckoned to be that large. Perhaps the coins were 1000 Galleon coins (wizards never get muggle matters straight and could easily think that had to pay such a ridiculously high amount). It would be foolish to think that wizards only had one galleon coins, one sickle coins, and one knut coins, though it can't be said that wizards are not foolish.
Author's response
- Yes, coinage in MODERN cultures is a figurative value. The Wizarding culture (and by extension, Goblin culture) is most specifically NOT a modern culture. Look at their 'Bank'. Piles of CASH is laying around in vaults instead of being invested. Insane. Given the militaristic mercantile nature of the Goblins portrayed in canon there is no way in hell the Goblins (and again by extension the Wizards) ever went off the Gold standard.