Does a shield or set of armor with spikes automatically have spikes of the same quality as the item to which they're attached? This question refers both to masterwork quality as well as special materials - if I have an adamantine set of full plate with spikes are my spikes adamantine automatically or do I have to spend an additional 3000 GP to have my spikes (a weapon) made out of adamantine?
You have to spend the extra gold. They're treated as separate items, in both crafting and enchanting.
I can't find an explicit ruling, but considering the entry below:
Armor Spikes: You can have spikes added to your armor, which allow you to deal extra piercing damage (see “spiked armor” on Table: Weapons) on a successful grapple attack. The spikes count as a martial weapon. If you are not proficient with them, you take a –4 penalty on grapple checks when you try to use them. You can also make a regular melee attack (or off-hand attack) with the spikes, and they count as a light weapon in this case. (You can't also make an attack with armor spikes if you have already made an attack with another off-hand weapon, and vice versa.) An enhancement bonus to a suit of armor does not improve the spikes' effectiveness, but the spikes can be made into magic weapons in their own right.
Emphasis is mine. In knowing that you have to add them to the armor separately, that they have their own item listing, and that they have to be magically enhanced separately, I believe that the intent is that you would need to spend the additional gold to have the spikes made out of adamantine.
Your mileage may vary, but I would rule that they should be paid for as though they were a completely separate item.
That's kind of what I was thinking, but I wanted to get the thoughts of others. Thanks for the input guys.
Well it depends. If your dwarves decide to menace the shield with spikes of platinum then you probably have a masterwork-worthy shield, but pig tails on the other hand...
Oh, wait. I just answered completely the wrong thing, didn't I? :p
On a more serious note, my feeling would be that you pay for masterwork spikes separate from a masterwork shield, especially since a masterwork weapon is twice as expensive. I don't see why you couldn't have a non-masterwork shield with masterwork spikes, or vice-versa.