SadisticFishing said:

It's because it's a lot easier to make a mithral chain shirt than full plate, by virtue of how hard it is relative to its thickness.

I included the cost for adamantium as well(x3) one would think that working with a harder material would increase the price but chain shirt 1,000 to full plate 9,000 is x9.

Nate, you are entitled to your opinion but could you cite some reasons based on the numbers I have presented, or your own.

Let's look at the mithral chain shirt. It allows one to use +2 more dex points that a normal chain shirt (+4 vs +6). Even if you are using a tough point buy system a rogue or finesse fighter will be able to gain a high dexterity. The additional points also help your armor check penalty reducing it to 0 with masterworked material. By comparison, a masterworked mithral chain shirt will cost 1,250gp. To get that AC with magical armor your would spend 4,250gp for +2. The magical armor would help in a small way be having more hp but the mithral quality would offset this as well. Also it would not help your touch AC. To get the same touch AC you would need a +2 ring of protection which would cost 8,000 gp.

Now before someone chimes in with but wait +4 gauntlets of dexterity cost 16,000 gp vs +3000 for magic armor and 8,000 for a ring. The gauntlets also help your reflex save and finesse attacks. The costs increase when you start talking magical mithral chain shirt.

A +2 magical mithral chain shirt would cost 5,250gp. A +4 chain shirt would cost 16,000gp while the difference between a +2 ring of protection and a +4 is 24,000gp. At higher level you will need epic items to cover the difference.

Mithral full plate is an even bigger issue because full plate grants the largest maximum armor bonus+dex bonus of any armor(+9). Also this allows classes that are designed to have armor limitations but other advantages, to bypass such limitations. The barbarian is a good example.

In an older post, (please excuse me for not having a link) the majority concensus was that an AC increase of +1 was better than a DR 1/-. Formulas were presented based on creatures players would fight once they achieved a level to buy adamantium armor or have DR as a class skill. Accepting this to be true then the cost of mithral and adamantium are not balanced.