A very interesting question - one that is probably hard to abstract an answer to. For instance, a kobold with a staff of power that can't use the staff has a great deal of wealth, but that just increases the loot, not the difficulty of the encounter. And even if it COULD use it, with crappy init, it could be taken down with one sleep spell, or one arrow, meaning that the wealth didn't amount to much. I think this is something a DM will likely just have to ballpark. But then one probably needs to do that anyway, from time to time, when it comes to EXP.