One thing to look out for: These are not "legal" to buy.
They were leased by Apple to the developer, with the strict condition, at time of lease, that they be returned when the lease was over (December 31, 2006.) There was no option to purchase outright. Apple did give developers even more incentive: If you returned the kit early (right after the production Intel iMac came out,) they would send you an iMac 100% free. (Well, realistically, you paid $999 to rent the DTK, then get an iMac to keep, so it wasn't *REALLY* free, but you get the point.) Therefore, unless the one out there was acquired from Apple's dumpster, it is technically "stolen property".
Of course, does Apple care enough to enforce this? I doubt it.
The actual "technical" value of the DTK is minimal. $50, tops. (Maybe $75 because of the case.) It has value solely as "the Apple-Intel transition computer".
And that valuation lies solely with you.
I have a friend who works at Intel, and helped develop that board. He didn't even know that it was for the Apple transition kit until just a couple months ago, when I found the printed Intel part number for it, and asked him to look it up. "Hey, I worked on that. Some custom development board for a random OEM." I told him the random OEM was Apple, and his reply was "Huh. That explains a lot."
It came with the maximum processor it was capable of running. No Core 2 support, sorry.