Pics and info here http://members.ispwest.com/gregg1/
You don't see a Yeager Prototype Duo 280 everyday, or ever. I've had these for a while and searched long and hard online. I've not found any mention of another prototype or engineering sample Duo in private hands.
The 230 has a 240 meg drive in it now. I just tried powering them up tonight and it appears the drive in the 280 has died in storage. The screen lights up with the pointer but the drive goes "whirrrRRRRR-clickety-RRRRrrrr" and repeats that over and over.
The installed system info on my site is outdated, I loaned them to a friend for a while and he redid the software to his liking. (Neither had their original drives or software when I got them, so nothing historical lost there.)
Other than the two Duos, I have a Duo Dock 1 with a NuBus ethernet card, ONE Duo AC adaptor, ONE Type 1 battery and ONE Type 3 battery. The 280 doesn't appear to want to power on with the Type 3 battery inserted. It used to but hey, it's a prototype, it can do whatever it wants. It works with the Type 1 and the 230 works with either, thanks to the Type 3 battery extention.
As to value, I have no idea, but they should run more than the garden variety Duo 280 or 230.
If you're into trades, I'd love to have a Beige G3, desktop or tower, with the Revision C mainboard and ROM. CPU as fast as I can get. 
I'd take both if we can find a good price.
msg me or mail me to till [at] kressline [dot] de
i am alos interested in the duo's
sent you a PM