I've always used the price given in the book as the base average for a middle class city. I adjust the price for what I feel the demand is for the locale, how much coinage/loot the party has, and how I feel about the group having a particular item. If it's not going to impact game play too much, then the cost isn't an issue. If it's something significant, I make sure their purse strings feel it. Coinage/loot isn't really a determining factor for our games. If the GM wants the group to have more money, it's easy enough to do when the loot the bodies of the dead, pickpockets in crowds, or burglarize a house. You can award as much loot as you choose. I guess for us, gold is irrelevant. In one campaign the group had so much loot, I made them account for the weight of all the gold they were carrying and they couldn't carry it all. They were leaving silver behind, much gold, and looking for only gems to carry, no where near a city to spend it.
I've never let them flat out purchase something stellar like a +25 Holy Battle Axe that does an additional "C" Cold crit on impact. No no no.... they have to EARN that one