Well, that's quite true. I don't know if that's a problem though, as NPC market and player market are two different realities. So what actually matters for the player is what market dominates the good he wants information about? Gold is undoubtedly dominated by the NPC market, as it's and end product with no use. But steel, as an unfinished product with use to the crafters, it belongs to the player market, in which it's price goes around 2000 trias (making each iron and coal piece be around 200 trias).
I guess the way to solve it would be: in reality, what has more of the product? Do five gold ingots have more gold than a gold stock? Cause that's what you're buying, right? The quantity of the good (I'd add quality, but that doesn't matter at all for the NPC market, and in steel stock has little relevancy) and not the effort someone had doing it.