I general, at least two thirds the cost of anything is the labor, so even when an item made of mithral costs 500 g.p. per lb. do not assume that the mithral itself costs 500 g.p. per lb. Probably the true price for unworked mithral bars and rods is somewhere between 100 g.p. and 200 g.p. per pound (depending largely on whos buying). I'd guess 100 g.p. myself, making it an even progression up from p.p. (ei 100 g.p. = 10 p.p. = 1 m.p)
If we choose a higher number for the cost unworked cost, it would not work correctly as the cost of mithril armor. For instance, the cost of making mithral banded mail is equal to the cost of making masterwork mail plus the cost of making it out of mirthral. Mithral banded mail costs 9250 g.p. and weighs 18 lbs. We will assume that the mithral version is essentially all metal because mithral chain is so light that leather is used only for straps at most. If we make the unworked cost 500 g.p., then > 9000 g.p. of the cost is in the material. This is less than the masterwork surcharge. The situation is worse for mithral full plate costing 10500 g.p. and weighing 25 lbs. because the cost of the raw materials is greater than the cost of the armor.
So I'm guessing that the armorer only pays about 100 g.p. from his supplier for mithral. If an adventurer comes in with 25 lbs. of mithral he picked up in the Tomb of Horrors and wants a suit of masterwork mithral plate mail made, it only drops the price from 10700 g.p. to 8200 g.p. Most of the price is in the (highly skilled) labor, although the armorer, being a sales man, is likely to complain about the cost of obtaining rare oils for quenching and the huge amount of charcoal requried to heat mithral and so forth to disguise what should be obvious - not many people can work mithral and those that can expect to make a decent living.
Just as a guess, I'd say that the raw material cost of adamintium is similar, but that because it is so much harder to work with (literally), the final cost of goods is slightly higher.
Oh, and for the record, an Expert (Blacksmith) with +19 in craft blacksmith working by himself takes about 10 years to make a suit of mithral heavy armor. Considering that historically a suit of steel full plate took about a year to produce, that's not that unrealistic.