I can't help you on 400KW exactly, however I can tell you that I saw a pair of 230KW industrial Onans sell for $6,000 - $7,000 each on GL near here last fall, the cheaper of the 2 showed 7,000+ hours on he meter, the more expensive was slightly newer same model with digital controller and unknown hours. I have seen the same model also show up with lower hours sell for over $11,0000, but have not closely paid attention to the exact ending prices. These prices were all for units that appeared cosmetically intact, I suspect the 400KW units would sell for proportionally higher amounts, although they rarely show up around here as surplus generators tend to fall into 2 generally groups, those that are small enough for individual use, say less than about 40KW and ones that are too large for individual use, say over about 40KW. The result is sort of a double price curve with the highest prices usually going to the sub 20KW units (I saw some very nice 10 year old low hour 12-15KW Onan commercial units recently sell in the $5,000+ range a couple of months ago) , dipping down again to a low in the 50KW range, then climbing back slowly for the larger industrial models as the majority of end users of these sets would never buy AS IS, these same users have traditionally avoided even buying used, so even resellers are hesitant to bid too much.
Ike
p.s. for one more data point for you, I bought a 30 year old very low hour, enclosed, 125KW Onan 125.0DYD off GL about 2 years ago for $4,000 for the family wholesale food business, it took about another $500 in parts, and a bit of tinkering to get running (biggest expense was a new controller board at over $300). Before winning this one, I watched several in the 125-150KW range sell for $6,000+, including the slightly newer 150KW Onan that was sitting right next to it. In retrospect see why that 150KW sold for so much more, it was just enough newer to get the controller that was introduced in the mid-1980's that was still in production a couple of years ago, had an engine with better parts availability, etc.