D20:Masterwork Armor Rules

Just as with weapons, you can purchase or craft masterwork versions of armor or shields. Such a well-made item functions like the normal version, except that its armor check penalty is lessened by 1.
A masterwork suit of armor or shield costs an extra 150 gp over and above the normal cost for that type of armor or shield.
The masterwork quality of a suit of armor or shield never provides a bonus on attack or damage rolls, even if the armor or shield is used as a weapon.
All magic armors and shields are automatically considered to be of masterwork quality.
You can?t add the masterwork quality to armor or a shield after it is created; it must be crafted as a masterwork item.

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System Reference Document -> Equipment (SRD)