Lord Vukodlak |
The costs aren't arbitrary they're made for simplicity and balance. Making a longsword out of adamantine has the exact same effect as making a warhammer adamantine. The weapon ignores hardness.
So for weapons the special material cost is flat because the effects are flat. Its the same reason masterwork is a flat cost. In the case of armor, special materials tend to have more dramatic effects on heavier armor then on light.