nobodez
Explorer
Well, I think this has already been answered (and it is RAW).
FAQ said:On page 217 of the DUNGEON MASTER’s Guide, it states that “the cost of the masterwork quality and any magical enhancement remains the same” regardless of a creature’s unusual size or shape. Adamantine and mithral both state that items made from such material are masterwork quality and the masterwork cost is part of the material cost listed. How much would a set of mithral chainmail barding for my warhorse cost?
A strict reading of the rules indicates that in such a case the masterwork cost for armor should be subtracted from the special material price before applying the multiplier for any unusual size or shape of the intended wearer.
For example, according to the Armor for Unusual Creatures chart on page 123 of the Player’s Handbook, a set of chainmail barding for a warhorse (a Large nonhumanoid creature) would normally cost 600 gp (four times the normal cost of 150 gp). A set of mithral chainmail barding would cost 16,000 gp: 600 gp
for the chainmail barding plus 15,400 gp (the cost of mithral medium armor [4,000 gp], minus the masterwork cost for armor [150 gp], times 4).