limitation: one city

[lord buss]

i want to build a power that based on spell, that lays on one specific city.
im not sure how to price it. i dont know what percent of the world this city's size is, and i dont think that it is the right why to price it.
the limitation on the spirit of place powers is (Only in home area, -10%). but this price is low because the spirit is already lives only in one place and almost cant leave it.
the limitation on priest of city god powers is -20%. im not sure if it includes the sense of duty. its low sanctity outside the city, so basically -5 all the time.
druidic power modifier is -10%. the penalty is -3 for cities, -5 for pollution, and even in absolutly poisoned and empty places it works, although the penalty is huge. so, what do you say, what the modifier should be if the ability (mostly regeneration), dont work outside the city at all? -20%, -30%, -40%.