Mic Pre Kit Roundup...
PAiA Tube Mic Pre
PAiA makes several t00b-based products, including a mic pre that can be built in either mono or stereo configurations. $68.85 for each channel, and $29.75 for a rack that holds a pair.
Pat's Discrete Millenia Pre-Amp
The page is HTML c. 1994, blink tags and all, and a little bit awe-inspiring in its grotesqueness, but Pat does indeed sell a mic pre kit. Or part of a kit. Or something. I'll tell you what: if I'm gonna drop six bangers on something I have to build myself, I'm gonna want, like, a little more confidence in the seller, you know? Especially when I'm buying what is basically a couple trannies and a piece of perfboard. Caveat emptor.
Hamptone HJFP2 & HVTP2
This is more like it. Designed by Scott Hampton, this pair of kits (one solid state JFET-based and one tube-based) is really nice. The enclosure is even pretty sweet, when all is said and done. Not cheap at all, but you gets what you pays for. $549 for the stereo JFET and $699 for the stereo tube, available here. Scott will also build 'em for you, for a couple hundred more, which might not be a bad angle to look at, since this project is not for beginners.
Seventh Circle Audio
If you head on over to the Seventh Circle Audio page, you'll be greeted with one of the nicest kit packages. There are four different mic pres, based upon the API 312, Neve 1272, and two custom designs. There's also a rack that will hold eight of the units, so you can mix and match. Obviously, this is more of a long-term project, what with having to build 8 of the little fuckers. But price-wise, it's a good system. The pres themselves range from $149 to $324, and the rack and PS will set you back $500.
EDIT: As is noted in the comments, you get the power supply free if you buy a pre kit and the rack. I've changed my mind and decided this is The Best Deal On The Internet if you want to build your own mic pres.
Not a kit, as such, but a well-worn path for DIY. This is another Prodigy-Pro project, and there is a wealth of information in a meta thread on that forum about this. A pretty simple build; costs will vary with your choice of components and racking, but figure about $150 a channel or thereabouts.
JLM Audio Micro 1290, et al
This one is about as easy as a kit can get for the rare-air high quality shiznit. At the JLM site you can buy the kit for $250, and about half of it is already built for you. You only need to solder 36 parts to the board. This is based on the mic pre section of the Neve 1073. Lots of various options on this one, and JLM has a couple other offerings for the discerning kit builder.