If you're lucky, it's because the seller and or the miner both know that the stone costing several times as much have characteristics that would make it worth so much to a well educated and discerning user.

Any decent hard japanese natural stone can make a shaving edge that is pretty good. But the other however many % the rest is (the silky feel, the ease of use, and other non feel things like colors for rarity purposes)....that's going to cost some money.

The real reason being that the discerning users (and to some extent sometimes a prestige buyer who is just buying on price) will pay the price.