The Skirnir Magus Archetype gains an Arcane Bond (Su) shield at 1st level. The Wizard Arcane Bond (Ex or Sp) grants a masterwork version of that item.
So I get a free Masterwork Heavy Steel Shield. The Masterwork property only applies to ACP, not to shield bash attacks: "An enhancement bonus on a shield does not improve the effectiveness of a shield bash made with it, but the shield can be made into a magic weapon in its own right."
So, if I want to have it enchanted with a +1 enhancement bonus to hit and damage (like a normal +1 weapon) do I just pay 2,000 gp, or do I have to pay the difference in cost of the armor masterwork (150) and the weapon masterwork (300)? Or the full 300 for weapon masterwork? The Arcane Bond FAQ says "the cost value of the final item as listed in the item's statblock" but I'm not sure how that works with weaponizing a shield.
What about shield spikes? Table: Armor and Shields lists them as costing "+10 gp". Do you have to purchase a "Spiked Shield" costing (shieldcost+10gp) or can the spikes be purchased at any time and added to an existing shield? Can I buy Masterwork Shield Spikes for 310 gp and stick them on my arcane bond, turning it into a Masterwork Spiked Masterwork Heavy Steel Shield?
I think it would be easier to treat the spikes as the weapon, meaning I have to buy MW spikes (310gp) then enchant them like any other weapon, while I can enchant my existing MW shield as armor. But I'd like to have this all sorted out before I bushwack my next GM.
"An enhancement bonus on a shield does not improve the effectiveness of a shield bash made with it, but the shield can be made into a magic weapon in its own right."
While I regrettably am not an expert on this subject, I think this might refer to the Bashing property rather than the possibility of actually enchanting a shield as a weapon in addition the armor that it already is.
There was a post quite some time ago relating to shields as weapons/shield bash/off hand/etc.
One thing that would have been of interest to you from there, Bashing doesn't stack with a spike.
Otherwise:
1) You don't need a shield spike, the shield is a weapon and an armor at the same time if you choose to run both "sides" of enchants.
2) Book keeping will be easier if you choose to enchant the shield spike as a weapon and add it to the shield.
3) If you take Bashing and enchant the shield as a weapon, that first +1 enhancement bonus you add is wasted (they don't stack).
RAW the fact that it is a MW shield is probably enough to save you that 150gp difference in price between armor and weapon. It needs to be masterwork, no where does it say it needs to be masterwork (as weapon).
Ideally I believe the best damage comes from doing the Bashing enchantment (to get the double size increase for the damage die) and then enchanting the shield as a weapon. Again the first weapon +1 enhancement is "wasted" due to the bashing giving a +1 enhancement when doing a shield bash but the larger damage die is worth 2000 gold "lost" in the long run (at least to me). A shield spike only increases the damage die once. The type of damage B vs P might make a difference though as well when trying to make your mind up in a particular campaign.