http://www.bhphotovideo.com/bnh/controller/home?O=Search&A=details&Q=&sku=186977&is=REG&addedTroughType=search&BI=1824&KBID=2466
Check that out.
I've read before that for that model (it comes with the 600mm F4) they used carbon fiber instead of plastic. I can't find the reference now, I thought it was on a Canon site but I can't find it now. However I found a reference to the 500F4 using a carbon fiber hood as well.
So there's your price explanation.
I'm not saying it really costs that much to make something out of CF, I think carbon fiber has become a sort of designer material that manufacturers use to jack up the costs of things. It's great stuff, don't get me wrong.. I've been trying to figure out a way I can make things with it, I fly remote control airplanes and composite construction is.. well.. kind of like L glass to an RC pilot. I'm not joking, Canon users have their white lenses, RC people (RC cars, like real cars, use this too) have the black woven look of carbon fiber. There are some instances where manufacturers have covered non carbon fiber parts with a carbon fiber print, the look has become that much of a desire. But I'm not certain if it's the sort of thing one can cook up in the basement like fiberglass is, not without more specialized equipment, heated molds and such.
The thing is.. CF shouldn't be lighter than plastic, just stronger, but it was mentioned along with the lightweight construction of the lens as a way to save weight. You use it for applications demanding strength. It's beyond me why Canon decided to use it to make a lens hood. It's not like the lens hood is meant to hold any weight.
Or maybe it is. Maybe the intention was that the hood might take an accidental impact and protect the lens itself. With a lens that heavy the hood would have to take a lot of energy.. it's a weak guess, but it's all I have.