It occupies that odd little niche between magic item and construct.  I think in 2e they were 2,000 gp but the pricing seems a little off for 3e.  I did find a stat bloc after digging around a bit more at Planewalker but it seems a bit geared for using a mimir as a familiar (tiny construct with weapon finesse (bite)).  Not sure if 2e ever statted up the mimir implicitly as a magic item either but I'm leaning towards the 'mimir's are tape-recorders that can cross reference entries' rather than the portable flying sage camp.
So before I did any real work on it for my campaign to come up with something close, I figured I'd ask around
Anyway for completeness here's the anonymous monster entry I found at planewalker-- 
Thu, 01/01/2004 - 07:00 — Anonymous 
Tiny Construct
Hit Dice:	1/2d10 (2 hp)
Initiative:	+2 (Dex)
Speed:	Fly 40 ft. (perfect)
Armor Class:	14 (+2 size, +2 Dex)
Base Attack/Grapple:	0
Attack:	0
Full Attack:	Bite +4
Space/Reach:	2 1/2 ft. by 2 1/2 ft. / 0
Special Attacks:	None.
Special Qualities:	Construct qualities, hardness 10
Saves:	Fort +0, Ref +2, Will +0
Abilities:	Str 8, Dex 15, Con --, Int 10, Wis 10, Cha 10
Skills:	Knowledge (the planes): varies from +5 to +20, depending on quality. Also, mimirs always take 10 on Knowledge (the planes) checks. 
Feats:	Weapon Finesse (bite)
 	
Environment:	Any
Organization:	Solitary
Challenge Rating:	1/2
Treasure:	None
Alignment:	True Neutral
Advancement:	None
Level Adjustment:	0
 
A mimir looks like a large, stylized human skull made of metal. It is a magical item, but has stats like a creature. 
Combat
Mimirs can do very little damage, so they don't usually fight. One might, however, bite the hand of someone trying to steal it.
Habitat/Society
Mimirs are often sold in Sigil as handy sources of information about the planes. They usually speak Common, but can be programmed to speak a different language. A mimir can be used as a familiar for an arcane spellcaster; such a familiar grants its master a +1 bonus to all Knowledge skills the master knows while within 1 mile of the master. Information on creating mimirs as magical items will be added to the magic item section.0
Ecology
None.