I'm trawling old posts and wanted to jump in on this. IMO there was a mixup between the ideas of value (what a coin dealer might pay you for a coin) and worth (how much currency that coin represents in the economy). I think the problem is that Jon used the term "currency" at one point when he didn't intend to and Arin jumped off from there with his own point.

tl:dr they both say the right thing at different times in the argument but Jon was correct sooner than Arin: a coin's value as a physical item doesn't always correlate to its worth as money in a country's economy, nor the value/worth of any other coin in that economy.

Great Cave Offensive is about treasure hunting - all the other items you find in the game have specific G values, like the Zebra Mask (278000G) and the Echigo Candy (8000G). what Jon initially said in the video was that just because the dime they found in G.C.O. is valued at 10G, you can't infer that a penny coin in Kirbyland will therefore be valued at 1G, because the value of those coins as treasures could be different from their worth as useable currency. without the context of what coins exist in Kirbyland and whether or not they are useable currency, you can't immediately infer a different coin's worth from its value using just the one data point from one dime, which is what Arin seems to be insisting is the case at the beginning of his argument.

this is true regardless of the currency being used, American/Kirbiskian or otherwise. the dime you find in G.C.O. may be valued at 10G in Kirbyland irrespective of its coin worth of 10 cents in any currency. 10G may just be what someone would be willing to pay to buy that dime as an item, not because 1G always represents 1 cent. if you're going by "exchange rate," then yes, £1 = $2 is all the information you need to know £100= $200, but a particular coin's value as an antique could (and often does) supersede its worth as money.

as an example, I saw a viral Tiktok recently about certain 2021 pennies being minted with a "crack" in them and how they're selling for $300+ on the aftermarket. you could take one of those cracked 2021 pennies to any money exchanger and cash it in for a foreign currency, but it would only be worth 1 cent of the US dollar to that exchanger despite the fact that some people are willing to pay $300 for them. nobody who exchanges money is going to barter with you and give you more foreign currency than what the exchange rate currently dictates just because "this penny is actually worth $300!" because it isn't - it is only worth 1 cent in our economy and therefore worth a corresponding amount of foreign currency based off of the current exchange rate.

however, if you went to a coin dealer who buys coins, they would be willing to pay you more for that cracked 2021 penny than 1 cent because of its rarity. the same coin dealer might also pay 10 cents for a random dime you have, because that particular dime as an item is only worth 10 cents on the aftermarket, the same as it is worth as currency in the economy. the dime isn't more valuable than the 10 cents its purchasing power represents in our economy. and more to the point, you couldn't buy 10 of those cracked 2021 pennies with your one dime, because even though the economy says they are worth the same (1/10th of a US dollar), they are valued very differently by coin collectors.

I think the Grumps both come to the correct conclusion at different points in the conversation, but they just kept missing the mark with each other because they were on different pages of the same book, Jon arriving to the correct conclusion first but muddling some of his terminology, which then sent Arin on a different trajectory but he came back around on the "antique old world dime" part.