Engineering Sample should be exactly what it sounds like - a sample part from Motorola so you could build up a prototype machine. These aren't very common. I've seen a handful pop up on eBay every now and then. Usually accompanied with the obligatory "RARE!!!11 OMG! APPLE PROTOTYPE!!!111". They aren't common, but they aren't that rare or precious, either.

Sounds interesting. If you decide to part with it, I'd be happy to take it off your hands. I'm not quite certain what a fair market price would be, though.

Peace,
Drew
_________________
Power to the PowerPC!