I was reading in the Planewalker's Handbook that some magical items are made to function only on a particular plane, which makes complete sense to me. This got me to wondering, though: how much does that restriction reduce the cost of the item? I looked around for an answer in the book but couldn't find it. I mean, if an item only functions on Gehenna, is that worth more or less of a reduction in cost than an item that only functions for one alignment?
Magical Item Reductions in Cost
I am working under the premise that it will reduce the cost at least in some way. One reason for this is that while some folks will be willing to buy things full price (because they never intend to leave the plane), many folks will not. I will be running games for planeswalking characters that will not settle down. No doubt there will be haggling involved when they seek out equipment. I'm just wondering if this is addressed in the books anywhere, or in the updates via Planewalker.
The answer you seek lies in the DM Guide to the Planes booklet in your original campaign setting boxed set, and it's not the one you want to hear. Regarding the text from the Planewalker's Handbook allowing for a reduced price on items forged only to function on one plane, well, I certainly wouldn't discount things based on limited usefulness because everything is already of limited mulitiplanar usefulness.
"Most magical items are bound by the same rules and limitations as wizard spells when it comes to functioning on the planes. Take a wand of fire to the plane of Water and all it's really good for is making harmless bubbles of steam. A wand of wonder, which uses wild magic, is a useless stick on orderly Mechanus. And unlike spells, there are no keys to make magical items work properly." Paste in the rules for plusses dissolving as you cross planes, and there you have it.
This leaves you with two possible answers, either of which is brilliantly correct:
1) This one reference in the Planewalker's Handbook refers to a practice of magic that is no longer performed due to its inefficiency and complexity. -OR-
2) Discount it however you want in your campaign.
Are you playing 2nd or 3rd edition?
Personally? I'd say it doesn't affect price at all. The planes are big enough that finding someone who never has any intention of taking that Staff of Power out of Limbo and is absolutely willing to pay full market value for the device should be reasonably easy. Making an item usable only on a particular plane or by characters of a particular alignment is a DM tool for making something rare and unusual less exploitable by power-hungry PCs. Not anything that's likely to affect the balance of the campaign.