@Vincentius @kornboi @ShionSinX
ShionSin is correct. All these isekai that convert fantasy money to yen are straight-up retarded. That's not how money works. The value of things depends on so much - scarcity being one of the most. You cannot know an exchange rate by going "hmm, a copper coin buys a loaf of bread, and a loaf of bread is 100 yen, therefore 100 yen = 1 copper." It doesn't work that way.
First, for all we know, the MC might just happen to be in a town near where wheat is grown, and thus baked goods are cheap. Or he might be in a town where the only bakery burnt down last week and he's paying 10x as much as normal. The entire point of a merchant class is that things cost more in one location and less in another. So knowing the cost of a piece of bread or a night at an inn in one town does not tell you that's the universal cost - it doesn't even tell you the cost of an inn in THAT town, as there could be cheaper/more expensive options.
Second, there must be an exchange of currency for there to be an EXCHANGE RATE. (And of course, exchange rates fluctuate right here in the real world. 1 dollar = 100 yen isn't even true on Earth from day to day.
Third, a world's economy depends on its resources, which may or may not be similar to earth's resources. For all we know, the MC could have isekai'd himself into a world where diamonds are super plentiful and quartz is exceedingly rare, or where certain technologies and resources aren't readily available.
Fourth, there's all kinds of services and items for which the MC is just not going to have any kind of frame of reference. Let's say he wants to buy a new sword. Does the MC know how much a hand-forged longsword costs in Japan? Why would we expect there to be a 1:1 ratio of costs of longswords or wagon wheels or buggy whips or horses just because he thinks bread is similarly priced?
To use a real-world idiom, "What does that have to do with the price of tea in China?"