Her's an interesting note... compare the
SRD entries for Adamantine, Mithral with Darkwood.
Adamantine: "Weapons and armor fashioned from adamantine are treated as masterwork items with regard to creation times, but the masterwork quality does not affect the enhancement bonus of weapons or the armor check penalty of armor."
Cost is dependant on the item.
Mithral: "Weapons or armors fashioned from mithral are treated as masterwork items with regard to creation times, but the masterwork quality does not affect the enhancement bonus of weapons or the armor check penalty of armor."
Again, cost is dependant on the item.
Darkwood: "Any wooden or mostly wooden item (such as a bow, an arrow, or a spear) made from darkwood is considered a masterwork item and weighs only half as much as a normal wooden item of that type."
Cost is is the price of a masterwork item + (original weight)x10gp
So...
Adamantine and Mithral count as MW only for the creation time, meaning you
must make a MW adamantine sword or mithral breastplate to enchant it. Neither material includes the 300gp MW fee in its creation costs.
Darkwood, on the other hand, counts fully as a MW item in all respects, and includes the MW cost in the base price of the item.