I always go to extremes when discussing use cases.Anything entirely in-game is fluff to me. You can change the price of a spear to anything you want, and most games don't break. A game that revolves around acquiring better gear, with rules that depend on that idea, would have some official equipment-price-rules. D&D's rules about what gear a character starts with are very much guidelines; DMs can add to it, reduce it, allow random rolls, etc, without breaking the game (other rules). There are actual rules about types of gear. "Heavy armor" has certain rules and rule-interactions, but I can make up 8 new varieties of heavy armor and the rest of the rules remain completely intact.
+4 vorpal sword - 10sp, and easy to find.
Or other: Potion of Greater Healing, 2gp, and easy to find.
Feels like these might break a game?
As such, is the price on these a rule? Or merely fluff?