There are some definate ways this could work, but you have to balance the xp cost with the effect.
Ex1:
Wizard crafts a Scroll of Limited Wish, but instead of investing the 300xp during creation, makes it such that whomever uses the scroll pays the 300xp.
-Seems balanced
From there it's not a large stretch to an item that casts Limited Wish x times per day/week/whatever but requires the person using it to spend 300xp.
-Still seems balanced
You could even have more wacky (if dangerous and potentially unbalancing) effects.
Accurate Sword
An 'Accurate Sword' never misses it's target. If an attack roll would result in a miss, the wielder immediately gains a competence bonus to attack sufficient to allow the attack to succeed by expending 20xbonus XP.
Ex1:
Wizard crafts a Scroll of Limited Wish, but instead of investing the 300xp during creation, makes it such that whomever uses the scroll pays the 300xp.
-Seems balanced
From there it's not a large stretch to an item that casts Limited Wish x times per day/week/whatever but requires the person using it to spend 300xp.
-Still seems balanced
You could even have more wacky (if dangerous and potentially unbalancing) effects.
Accurate Sword
An 'Accurate Sword' never misses it's target. If an attack roll would result in a miss, the wielder immediately gains a competence bonus to attack sufficient to allow the attack to succeed by expending 20xbonus XP.