In the text, if a focus item can only be used to augment a particular spell, it is treated as having an extra item slot worth of power. So I suppose it is reasonable to give it a similar bonus for augmenting a spell versus a particular target.
So, "Outsiders bane +2 offensive power (whatever your favorite element is) +2 offensive control, only for attacking outsiders" would cost two item slots.
Alternatively, you could make a defensive spell which has an aspect requirement "only versus outsiders" and then an item that only boosts the power of that spell.