Hit the FAQ.
But just to throw another bone in the pile.
Mithril is generally priced by weight for non armor/weapon items.
Would it make sense for the size/weight of the armor to take a similar route...
A mithril chain shirt for a human is 1100 gp
A mithril chain shirt for a medium sized small cat has the exact same weight. Sure, it costs a little more to make, due to the odd shape, but the amount of mithril present has not really changed.
So wouldn't it make sense for it to be 200*2 + 1000 = 1200 gp?
I can't see why you'd double the mithril cost if there's the same amount of mithril in it.
That would mean a chain shirt for a large-sized animal, which only weighs twice as much, would end up 100*4 + 1000*2 = 2400 (instead of 4400 if you'd multiplied it all by 4)
So basicaly, multiply the base armor price by the cost multiplier (x2 for medium, x4 for large) and the mithril price by the weight multiplier (no change for medium, x2 for large)
That makes sense from a materials standpoint, and might keep the armor in a price range where folks might be able to afford them.
Of course, it then makes you ponder small-sized guys...
Just a thought.