I know there has been some questions posed in regards to iSupply's estimation methods for Wii production costs so I wouldn't be surprised if the same issues were carried over here as well. In the past we have seen:
actual cost = O(iSupply estimate)
In short, iSupply's estimate is probably a good upper limit estimate, but probably not a very good ideal estimate (Ω) or realistic estimate(Θ).
There are a lot of difference between Wii and PS3 production though, so who knows.