Magic Item Pricing
Since I was one of the architects of the Dungeon Master’s Guide’s magic item pricing system, it should come as no surprise that I’m a big supporter of using those guidelines to gauge the appropriate price of a magic item.
Even then, we knew that no system could properly account for the vast range of magic items possible. Any system—regardless of complexity—would have to rely on the DM’s best judgment.
And while it’s clear that most players out there understand that, there’s still a fair number of gamers who either don’t understand how the system works or want it to be something it isn’t.
As stated in the Dungeon Master’s Guide, the various equations and systems described in Chapter 8 are only guidelines. Except for the simplest of items (potions, scrolls, and wands), the DM must be involved when attempting to price any new magic item. Even two spells of the same level, when turned into a magic item, may result in radically different appropriate prices. There are a few reasons why this is true.
First of all, not all spells are created equal, even those of the same level. Some spells are just better than others, particularly when their effects can be applied at will or continuously. Compare the ability to cast either of these spells “at will”: knock and invisibility. Chance are, you’ll only need knock a couple of times in any given adventure. Having an “at-will” knock magic item isn’t really significantly better than having a few scrolls of knock. But invisibility can come in useful in almost every single encounter, and often several times even in the same encounter. (You could also argue that invisibility might be too powerful to be a second-level spell, but that’s another discussion.) Thus, even though the “system” would suggest that these two items cost a similar amount (about 12,000 gp), it’s clear that they really aren’t “worth” the same amount. That’s why the ring of invisibility costs 20,000 gp in the Dungeon Master’s Guide (and may be undercosted at that). Don’t be afraid to adjust a magic item’s price by one-quarter, one-half, or more in either direction, based on the game utility of the spell.
Second, some spell effects are balanced by their limited duration. Both displacement and fly are incredibly useful third-level wizard spells, so shouldn’t both cost the same as continuously active magic items? Of course not. Displacement lasts only a matter of rounds per casting, while fly lasts for minutes if not hours. You’d have to cast a hundred displacement spells in order for the effect to last as long as a single fly spell. That’s why a cloak of displacement (the major kind) costs 50,000 gp—far more than the expected value of 30,000 gp for a magic item replicating a third-level spell. As a rule of thumb, you can probably double (at least) the price of any magic item that provides a continuous magic effect based on a short-duration (1 round/level or even 1 minute/level) spell.
Third, some effects are balanced by limitations on who can cast them and whom they can target. True strike, for instance, is only granted to wizards and may only be targeted upon the caster herself. So while equipping every character in the party with bracers of true strike might seem a great idea (only 2,000 gp each, right?), the DM should quickly realize that such items effectively bypass the built-in limitations of the spell itself. In this case, you might try to decide what the appropriate level would be for a true strike-type spell that could be cast on anyone (not just the caster) and price the item accordingly. My off-hand guess is that it should be at least a 3rd-level spell, meaning that those bracers of true strike should be somewhere in the neighborhood of 30,000 gp.
The magic item pricing guidelines can be a powerful tool in the hands of a DM who is comfortable using them, but they can be an equally powerful weapon in the hands of overeager players who, intentionally or not, use them to abuse the game. As the DM, you shouldn’t ever be afraid to say “no” to your players (though it’s best to explain why you’re saying it). Only after you’re acclimated to the game should you start tinkering around with new magic items, and even then, don’t hesitate to come down on the high side when pricing the item. After all, you can always give the character who purchased or created the item a refund of gp and/or XP, but it’s far harder to ask for more payment after the fact.