Actually, this is a question that is addressed in the FAQ, page 62 to be specific.
Except that FAQs are lulz.
Their accuracy for rules is lulzy in the extreme, and it's not RAW in any case. That said, in this case they have the right answer, but taking them seriously is, in general, a bad idea.
Is the Monk's Belt overpowered for the price? Yes and no.
No, it's not broken period.
It's worth a lot to a character with a Wisdom of 20, more if the Wisdom is higher.
No, it's worth almost nothing to most high wis characters because clerics should be wearing armor and druids have Wilding Clasps so they can have better equipment as well.
But it is what it is. Nobody with a low Wisdom should waste their money on one.
And just because no non-caster can use the Karma bead, I guess it's utterly unproblematic.
Heh. Yeah. I was trying to figure out how to get a background color, then figured just to forget it.
Though, technically, in Pathfinder, it's a Monk's Robe. In some games, you could end up wearing both! (In my own games, I'd probably just allow one or the other in terms of form factor, and follow the rules of the game I'm running.)
And wearing them both would be meaningless as they both grant the same ability, and so don't stack.
It is, admittedly, only overpowered in limited circumstances. But within those circumstances, it's pretty wildly overpowered.
Only if those circumstances are "never" or "under no."
That said, even most clerics would probably be better off in armor than with a Monk's Belt.
And it can't take armor enhancements. It's terrible for most clerics, and a cleric can pump its AC so high anyways that even if it were to offer some gain, despite, you know, being almost never superior to +5 plate, it's generally irrelevant.
A druid could make excellent use of one though, as their armor choices are more limited...especially if you take Wildshape into account.
No, they really aren't.
A ranger...kind of depends. I think overall they'd be better off with armor, but you could probably do a dex/wis specialist build that'd make good use of a belt.
No, a ranger will never be better off. Or, rather, if they are the build is so utterly terrible that it's pointless.