Breaking down the costs it doesn't seem so outrageous. I think it goes something like this:
$575: nVidia 7900 GTX 512 (about the going rate for a top of the line mobile GPU; in this case, *the* top of the line mobile GPU)
$100: New power brick (I know I know, If you've already got a 220w brick with your 9880, why do you need a new one? That's another rant entirely.)
$150: New mobo (not bad I suppose for a 915-based mobil mobo, plus you get a shiny new 1 yr warranty on it)
$75 Labor and shipping (doesn't seem too bad <shrug>)
As far as keeping parts that you could sell on eBay, well I could be wrong but I don't think the market for used Clevo mobos is all that hot. I'd just as soon Sager keep them, give me a few bucks, and save me the eBay hassle. Same for the GPU since it can only be used in Clevos.
But that's just my opinion and others often disagree with me (note my rep).