That is a tough one, these are current generation models and don't show up on the surplus market very often. I can tell you that the previous generation of 3KW units built in the late 1980's typically sell in the $400-$600 range, this seems to be true of both the 28V DC units and the 60 HZ selectable voltage AC units, with the AC units being much more commonly available. The MEP-501 is much lighter than the older models, and likely has much better fuel economy.
Ike
p.s. some people have expressed concern about maintaining these newer models given their increased use of speciality electronics.