To come up with a specific dollar amount like that makes me wonder. Even if iSupply gets exact details on what each part "costs" Sony to purchase, they still have to figure in shipping costs, finance charges if Sony uses any sort of credit to buy in bulk, overhead costs (electricity, health insurance, workman's comp insurance, etc etc etc).
So the reality is, that if the "per part" costs are the $336.27 they claim, then they've still off by a bit. Who knows, maybe iSupply had an inside source and was able to get input from someone at Sony who had their "estimated" prices as to what something "costs" them including overhead breakdown and such.
Either way, I still think the PS3 and XBOX360 are more expensive than what I as a consumer think they are worth. Which is why I haven't purchased either one. It's all about perception though. Many console gamers would say my gaming PC isn't worth it either. So it's all in how you look at it.