For a specific setting? Specific timeframe and region in that setting? How is it going to be any more accurate when it's all just make believe? Having more details and explanation doesn't necessarily make it any more accurate.
Because, as
@Crimson Longinus pointed out, there is no reason for
anything to have a consistent price. How much is a gold piece worth? What's the level of technology for making steel for that sword or armor? Does the world have automatons that can help refine the metal? Is there a good source of iron (or other requires metals) available? Do they have a way of harnessing fire elementals
The cost should vary not only by campaign but by region and even things like social class and connections.
The other aspect of needing to spend 100 gp on something is that magic does not have to be based on science or alchemy. It can be based on symbolism, effort, willpower and sacrifice. It can just be that you need to sacrifice 100 gp because of the amount you are sacrificing. It has nothing to do with the physical amount being used.
That, and D&D is a game about play pointy eared elves and slaying literal dragons.