Is the cost of specific material barding before or after any multipliers?


I am looking at buying Mithral barding for an animal companion. It is a large, non-humanoid creature. I want to buy it full plate. Would the cost be 1500 (base cost of full plate) × 4 (large, nonhumanion surcharge) + 9000 (Mithral surcharge) = 15000 gp or (1500 + 9000) × 4 = 42000 gp?