I imagine there's a few things going on that are inflating the value of items. First, from the light novel:
A hundred Gold is more than enough to buy a single loaf of bread.
It’s not quite wrong to say that 1 Gold is equal to 1 Yen.
So, first thing going on is that 1 Gold = 1 Yen in value, roughly. So that sword would be worth roughly $383,360.85 in USD, based on the exchange rate right now. Still expensive, but not nearly as much as $51.5 million USD would be. There's also nothing smaller than 1 gold, like there's nothing smaller than 1 Yen in Japan. With that said, it's likely that there are coins of varying values, given that in Ch10 of the manga, he can pay for 3 healing potions that cost 72,000 gold from a bag that looks like it'd barely fit the three of them together.
Second thing is that this is originally a game world. To my knowledge, there's been no explanation for where the coins come from, whether it be a mint, or specific dungeons rewarding in straight money, or if there's a way to convert items into money. Regardless, game worlds are rather notorious for severe inflation. In several games I've played, getting or maintaining housing is cheaper than getting decent gear well before endgame. I also highly doubt that people are farming dungeon materials to build their houses, get food, or whatnot, so day-to-day expenses are likely trivial compared to weapon/armor/skill book expenses. Note the prices for an "expensive" diner. And how Luce freaks out about the price, despite having just earned more than 1,000 times more gold
just for the subjugation of the Dream Lord in the first dungeon she conquers with Elma.
“Uwah! Crab King Pie with Black Mushroom Sauce, for 8000 gold! I almost ordered it just now, but I didn’t think it would cost that much… . Elma-san, things on this page are no good! Let’s order something that’s no more than 1400 gold!”
Finally, with the game world turned real, there's no revivals if you get killed.
Any gear, or items that could be used to make or enhance gear, that would let you fight better or survive longer would get dramatically more expensive. Notably, the status window marks the [Market Value]. Not the original price from when it was a game, but an average of how much the item would cost to purchase from a fairly-priced store in the region. From when he got the Mad Demon Shield:
I gulped.
It has the same stats as the one in [Magic World], but the market value is very much different.