This chapter...MAKES NO SENSE! Even if you ignore the nutritional value (because magic...okay) it was still a vital food preservative. It wasn't until the invention of the refrigerator did that really change in our world. Unless this world has the same, it doesn't make sense it'd be seen as a purely luxury item, and even if it was, there wouldn't be an inflation of prices!
Also, the MC's goal is to LOWER the price of salt, and he's going to do that by INCREASING DEMAND! That isn't how economics work! Even if their country was landlocked, the coastal countries would still be selling harvested salt for low profit. The basic principles of competition would keep it from inflating or depressing too far in either direction, so an outrageous price of 10,000 yen per kilo doesn't make any sense.
This is a very stupid chapter from an author who doesn't really understand what he's writing about. DO RESEARCH!