Cast steel weighs 490 lbs/cu ft, and rolled steel weighs 495 lbs/cu ft. Check it out
here. Taking an average, that would put 1350 cubic feet of adamantine (the same weight as steel) at 664,875 lbs.
Because all adamantine armor of each class incurs the same extra cost (1/3 market cost for materials) even though their weights are different, finding an average price per pound of all core metal armors made of adamantine would seem to be a reasonable method of finding a base cost per pound of adamantine:
Light Armor market price: +5,000 gp
Medium Armor MP: +10,000 gp
Heavy Armor MP: +15,000 gp
Chain Shirt: 25 lbs., 5,100 gp
Scale Mail: 30 lbs., 10,050 gp
Chainmail: 40 lbs., 10,150 gp
Breastplate: 30 lbs., 10,200 gp
Splint Mail: 45 lbs., 15,200 gp
Banded Mail: 35 lbs., 15,250 gp
Half Plate: 50 lbs., 15,600 gp
Full Plate: 50 lbs., 16,500
Total Weight: 305 lbs.
Total Market Value: 98,050 gp
Average cost of raw materials (per pound): 98,050/(3*305)= roughly 107.16 gp.
Of course, the pricing system is kind of inconsistent by weight, and one could expand the equation to include the prices of weapons and shields as well, but according to this formula,
1350 cubic feet of adamantine would weigh 664,875 lbs. and cost approximately 71,248,005 gp.
