Sometimes, a player doesn't want an entirely new item, just something with a few alterations. For example, a player with a monk character would love to have an amulet of mighty fists, but she has grown to depend on the extra protection she gets from her amulet of natural armor. So, the inevitable question arises: Does an amulet of mighty fists have to be an amulet? The answer is of course not! However, that begs another question: If not an amulet, what kind of item should it be?
The Body Slot Affinities sidebar on page 288 in the Dungeon Master's Guide can help answer the second question. A look at the table there shows that bracers (combat), gauntlets (destructive power), or even a belt (physical improvement) are the most appropriate alternatives. Considering that our example monk wants to pound foes with her fists and that she probably already owns bracers of armor, gauntlets probably are the best bet. So, what should the gauntlets cost?
You can reasonably assume that a pair of gauntlets of mighty fists ought to have the same base cost and market price as the amulet (6,000 to 150,000 gp), depending on the enhancement bonus the item provides. The +1 version (6,000 gp) has a monetary cost to create of 3,000 gp (half the base price) and an experience cost of 240 XP (1/25th the base cost).
On the other hand, a +1 magic weapon (such as a +1 spiked gauntlet) costs only 2,000 gp (for the magical enhancement). Another quick look at the table on page 288 of the Dungeon Master's Guide shows that the amulet slot is best for items that involve protection and discernment, not attack, so it's a good bet that the amulet's price already had an adjustment for an uncustomary item slot. That makes sense, because almost any creature can wear an amulet or necklace and the amulet works on unarmed attacks and natural weaponry. The amulet of mighty fists probably also is intended for familiars and animal companions as well as monks.
That uncustomary item slot adjustment is x 1.5 (see Table 7-33), so the base cost and market price for the gauntlets would be 4,000 gp (6,000/1.5). The item should have the same caster level as a magic weapon with the same enhancement bonus (3 x the bonus), and it would require the Craft Wondrous item feat.
Okay, what happens if the character in question already has magic gauntlets, too, or just wants to keep that glove/gauntlet item slot available for some other useful item, such as gauntlets of ogre power or gloves of Dexterity? That's not a problem, either. There's no reason why someone could not create a robe of mighty fists, or a vest of mighty fists. Such items should have the same cost as the amulet (remember that we're assuming the amulet already has a cost increase of 50% for an uncustomary slot), so a +1 vest or robe of mighty fists would have a market price of 6,000 gp.
DMs who just aren't inclined to give the monk a break on such items might want to charge 6,000 gp for the amulet or gauntlets and 9,000 gp for the robe or the vest. I think that's excessive, and I suspect that most players would, too.