Value of Casting Items

[Anthony]

The obvious option is to not create it as a magic item, created it as a Gadget. Unfortunately, that involves rules that aren't part of DFRPG. It's also not a lot of points saved, it's can be stolen (probably half cost because the item only works people with the relevant PI; quick contest, so -15%) on 15 points of skills, resulting in a cost of [13].

Your other option is to just give a quirk or two for "needs a special holy item" (possibly two levels; 1 quirk for needing a holy item at all, 1 for needing a specific one).