Not the most broken, but Mage's Lucubration...

[Segev]

Stupid thought that probably has occurred and been beaten to death in other optimization threads, but mage's lucubration (http://www.d20srd.org/srd/spells/magesLucubration.htm) could theoretically be placed into a command-activated item usable every round for 118,800 gp. Since it's only useful to a wizard, you may as well apply the 30% cost reduction for "can only be used by a wizard." That brings it down to 83,160 gp. This is, obviously, a lot of gp, but...

It makes it so that a wizard can literally cast every spell he has prepared of 5th level or lower as often as he wants. Not, perhaps, in combat (unless he's willing to only do it every other round), but he could nova his 5th level and lower spells and then recover them all afterwards. A quick teleport away, a few rounds using this item, and a teleport back, and he's recharged, too.

[Flickerdart]

Yes, if your DM permits you to create such an item and prices it like this, it's powerful. But it bears repeating that the guidelines are not rules, and even within the guidelines, the DM is told to compare the item to an existing item to determine its cost. A daily-use item that allows you to recover a 5th level spell slot happens to be identical to a 25,000gp pearl of power. Therefore, an item that allows you to recover 14,400 5th level spell slots per day should cost 14400*25000 (and don't forget that nice little Epic multiplier).

[Segev]

Ah, good catch on the Pearl of Power similarity. Interetsingly, a 1/day mage's lucubration item, even without the 30% reduction, comes to 23,760 gp. Thus, this clearly falls into the same category as why you can't get an item of mage armor, but instead must buy bracers of armor.

[Cruiser1]

It makes it so that a wizard can literally cast every spell he has prepared of 5th level or lower as often as he wants. A quick teleport away, a few rounds using this item, and a teleport back, and he's recharged, too.
As others have said, your custom item and its price must be approved by the DM. However, with a little cheese you can still (ab)use Mage's Lucubration to get infinite spells without magic items:

Have your Wizard PrC into Spelldancer 1 so you can apply metamagic without spell slot increases. Take the feats Repeat Spell, Twin Spell, Alacritous Cogitation (so you can spontaneously cast a spell), and Versatile Spellcaster (which requires being able to spontaneously cast spells).

Now Spelldance up a Twin Repeat Mage's Lucubration. You cast one 6th level slot to get four 5th level slots back. Use Spelldance + Versatile Spellcaster to spend two of those 5th level slots to cast Twin Repeat Mage's Lucubration again to repeat the process. The end result is you can continually spend two 5th level slots to get four 5th level slots, so have infinite 5th level or below spells. :smallbiggrin:

A similar technique can be done at an earlier level, with the 4th level spell Mnemonic Enhancer, that restores or prepares a 3rd level spell.