Altering Prices (DMG)

Remember that the prices listed in the Player's Handbook are not absolute. There is no reason you can't raise or lower the price of any item on the equipment lists. Demand can increase or decrease a price. Different lands in your campaign may be known for specific goods, allowing them to charge more.
Even in the Middle Ages, Spain and the Middle East were known for steel, Germany for beer, France for wine, England for wool, and the Italy for armor. These reputations allowed higher prices to be charged for these goods, especially finished items.
Greed can also raise prices. Merchants live to make money, so they will normally charge what they can get away with. There were very few price controls or regulating agencies during these times.
Finally, adventurers tend to disrupt local economies, suddenly bringing in large amounts of cash. Merchants raise prices to match. Situations not unlike the Klondike gold rush develop, in which even the simplest items cost outrageous amounts. In short, don't be afraid to charge characters as much as you think you can get away with. If they don't like the prices, they'll find some way to let the merchant know of their dissatisfaction.