Smithing unlocks are tied to the cost of the weapon you smith and smelt, so they can just adjust the values. I agree that the values vary way too much, you can craft a 10k denar sword for the same material cost as a 600 denar spear.
Why do you have to unlock the exact same part across multiple weapons? A pommel on a mace is the same thing as a pommel on a sword, both stat-wise and cost-wise. Is that a bug? I sometimes see other weapons icons flash as if i unlocked a part, but it doesn't unlock anything.