Core rules for enchanting used to be in rules, but apparently a lot of (whiners) didn't feel that there WERE any core rules, and it's almost certain that a lot of people have elevated the Rules to some sort of divine status, such that the fact that the core rules are listed as guidelines is seen as making it less cannon. ... In spite of the very real fact that ALL the rules are guidelines, and this type of rule is indeed there, it just has cautions in it stating that you have to use *more* discretion than with normal rules. Now, since these dogmatic types see the Rules as infallible and from On High, the fact that they might have to use some individual discretion makes them embarrassed and uncomfortable to the point that they shove the whole thing into house rules, and demand that other people do likewise, pretending that there are no rules in the core books for doing these type of things.
Now, that being said, one of the cautions in those rules is against someone making a shield of faith item priced as you have there... as UltimaGabe said, you have to use the overriding section regarding deflection bonuses. Additionally, if you look at effects such as 'shadowed' on armor, you'll see that it uses the +50% additional cost rule, and I'm pretty sure that you should settle for that, as the alternative is to (as suggested above) use the +100% rule of slotless. Of course, this is not completely slotless, as you have to have it in your 'weapon slot', and there are precedents in the armor section for doing so.
Edit: And yes, this is definitely an area where you have to ask your GM.