Supply and demand.

An item is worth whatever a person is willing to pay for it.

If there's only one Dodore Doublet for sale on the server and there's 50 people wanting to buy it, the doublet will sell to the highest bidder. Since the game isn't a real auction you have to start the price high. If no one is willing to pay 15 million then the price is lowered to 14, 13 etc. until it reaches a price that the highest "bidder" is willing to pay. As it happens there are people willing to pay 15 million for it. Once those all have one the price will drop.

This is basic MMO economics and applies in the real world too somewhat.

I'm saying no more as I don't wish to be dragged into an argument.