How I hate the "fair prices" policy MCs always try to enforce without trying to understand why the prices are what they are: cities don't pop out of nowhere, so why the nearest one is at 80 km? Probably because it is too risky to stay near the dungeon: if he can afford to double the price is because only a few brave merchant are willing to travel a full day, in the middle of a desert, with the chance of demon attacks. If it really was only greed, everyone would try to get a piece of the cake and the prices would inevitably lower...