Magic, Custom & Masterwork Armor - Agile BreastPlate
When adding Mithral as Material Used, only adjusts Weight and Treat As One Category Lighter.
Should also decrease Spell Failure Chance by 10%, increase Max Dex Bonus by 2, decrease Armor Check penalties by 3, and should only increase price by 1000 GP(Item is Torso only, not full suit needing 4000 GP increase)
Ah, you must be using the piecemeal armor variant. That's a bit complicated.
The Spell Failure chance reduction and Max Dex changes only take effect if all pieces worn are of the same material AND it is a reduction from the Cumulative stats of all the armor pieces worn (which is the worst of any piece worn for spell failure and Max Dex), so they can't be reflected in the stats displayed for individual pieces on the Armor tab. Instead, the program must check for them and then adjust things down before applying the effects to the hero.
So, example character is a 5th level wizard with all 3 armor proficiency feats, 20 Str (so he won't get encumbered) and 20 Dex (for the AC bonus check). Add a normal Agile Breastplate torso piece and one made out of mithral.
Before equip anything, AC is 15 (+5 Dex), spell failure is 0%.
When normal agile breastplate is equipped AC is 19 (6 armor, 3 dex) and spell failure is 25% (You have to look on the Wizard tab to see the amount of spell failure applied)
When mithral agile breastplate is equipped AC is 21 (6 armor, 5 dex) and spell failure is 15% (Again, look at the wizard tab)
Now add a normal chain armor piece and a mithral chain armor piece
Mithral Agile Breastplate and normal Chain arm piece should get no reduction from mithral armor material since whole suit isn't made of mithral. Worst of all worn pieces is Max Dex +2 and 30% (from the arm piece). Equip them both and we see AC 19 (7 armor, 2 Dex) and 30% spell failure.
Mithral Agile Breastplate and mithral Chain armor piece has the same worsts (+2 Max Dex and 30% failure from the arm piece), but it should get the mithral reduction so we should see +4 Max Dex and 20% failure. Equip and what do we get? AC 21 (7 Armor, 4 Dex) and 20%.
The Plate Torso piece is a heavy armor Torso piece, so it is correctly costing 4000 gp. It only counts as medium armor when worn alone, not for cost purposes. Plus, it doesn't make sense that something that costs 4000 gp when you make it out of mithral in one system would only cost 1000 when you make it in this variant.