{Damn laptop overheated and shutdown when I was replying this morning...}
Unfortunately the Octane V12 w/ DCD and the dual 600 CPU module are rare enough that they epitomize the line, "whatever price the market will bear." Sometimes you can find them for a lower price, but it's completely unpredictable. And if it were just about the capabilities of the system, even the price you cite would be way overpriced. As it stands, both of these items usually sell for over $300 when they do pop up - as you saw... But then you usually do better financially if you're able to completely part out and sell the components than you would selling the whole thing.
To SAQ's point, the cheapest SGI is the one you already own. (I have to constantly remind myself of this...

) Now if you wanted to swap a cheap Octane for your loaded Octane because you need the cash, that's a decision only you can make. But if you don't need the cash, or aren't sure - consider how much time, effort and money it'll take if you want to recreate it later.
If you were thinking of adding a second Octane just to have two, well, consider what you said about not having time to do anything with the first one... Again, something I have to frequently remind myself of so that I don't, for example, head down to LA to grab one of those Onyx2 desksides that are available.
Hope there was something helpful in all that blather.
