Powerful Item that makes you a quasi-Caster?

[ImperatorK]

I know that it is cheaper and more practical to have multiple magic items. But what I'm interested in is a single non-epic, non-artifact item that would give a character powers comparable to a casters power. The easiest path to this would be just giving the wielder of this item virtual spellcasting. Something like "When you wear the Robe of The Ultra-Archmagi you gain spellcasting like a 20th level Wizard".
How would you price such an item? I assume this item would be too powerful, so if it isn't possible to make as an non-epic item, then what alternative would you suggest? Maybe a refluffed, powered up staff?
Basically I'm trying to create an item that would make a quasi/pseudo-caster from someone without any casting abilities, but without making him actually a caster.

[Fouredged Sword]

There are a spell that grant psionic powers, PP, and a manifester level. It would be rediculusly expensive, but in therory you could use the magic item creation rules to create a continuous item of it, granting continuous psionics.

For even more expensive shananagins create a infinate use item for unlimited PP.