I have created a Ring of Honest Dungeoneering, which grants the wearer +2 competence bonuses on multiple skills, but once a week it allows anyone within a 20′ radius to command-word-trigger a Zone of Truth with target wearer; obviously, this is detrimental to the wearer. Additionally I’d like to make it hard to remove, e.g. only with a limited wish, wish or miracle.
I cannot find any tables for adding these effects. How would the cost for these effects be calculated?
In response to the comments, it’s worth pointing out that I am talking about a curse-like effect, not a curse. The reason it’s only once a week, was to reduce the cost of the permanent spell by ¹⁄₃₅, so it would be affordable for the companions.
The core of the question remains unchanged, though; it is a question about how to adjudicate an items power by its monetary value (as pointed out by one of you, dear users).