
SlimGauge |

Figure everything (not including magical enhancements) as if it was for your standard medium creature.
100 Chainshirt
1000 Mithral light armor (includes masterwork cost)
1100 Mithral Chainshirt weighs 12.5 lbs
Now adjust for size
Cost x4 weight x2 (non-humanoid large)
4400 Mithral Chainshirt Barding for a large creature weighs 25 lbs.
If you don't do it this way, you end up with collossal mithral breastplates that can be melted down for more worth in raw mithral than they cost you to get.

asthyril |

but if you do that , it also mean that you can have a small mithril shirt (humanoid) for half the price or a tiny one at 1/10 of the price.
so both way is wrong :P
what i would do is 1400 but you cant melt it!
no, SlimGauge is correct, it costs 4400g, and you CAN get mithril armor cheaper for tiny(not small) or smaller creatures, look at the chart in the core rulebook. that's because it requires less mithril. it also only provides half the armor bonus (pixie sized full plate only give 4 ac)