Just to remind everyone as well, one of the reasons why she had to overprice the good she's selling even more is because of the difference between the JPY to the isekai world's Coin currency exchange and the converted price of goods.
During the early chapters, it is mentioned that 1 gold coin is worth 100.000 yen in purchasing power in the isekai, but the Wolf Fang's appraised the gold coin to be worth around 25.000 yen, only 1/4 of the perceived value at the isekai.
Since price will differ depending on the isekai's needs and wants, lets use food ingredients (say a pack of 6 eggs) as an example of goods that may have similar prices in both worlds.
value-wise, lets say that on earth, it costs 100 yen to buy a pack of 6 eggs. In the isekai, it would cost about 1 small silver coin (value conversion rate at the bottom of the post).
However, if I want to exchange 1 small silver coin (lets assume the appraisal value of silver coins is proportionally the same as gold coins), I would only get 25 yen in the real world.
If I buy a pack in the real world using the isekai currency, I would need to exchange 4 small silver coins in order to get the 100 yen needed to buy the pack of eggs.
Which means that in order to breakeven, I would need to sell the pack of eggs at 4 small silver coins even though the market value is only at 1 small silver coins.
Thankfully what Mitsuha is selling are mostly luxury goods (in terms of the isekai's standard anyway) so she can gorge the price even further to ensure that she profits, and still have enough capital to reinvest in more products to sell.
But at the very least, she needs to quadruple the prices of the goods she bought on earth just in order to breakeven, so I am not surprised that the guests are shocked at the goods being sold at the store.
note:
- 1 gold coin =100.000 yen
- 1 small gold coin = 10.000 yen
- 1 silver coin = 1.000 yen
- 1 small silver coin = 100 yen