Incidentally, the cost of that 4GB RAM upgrade is what I paid for my entire laptop, which has 4GB of RAM. Admittedly, DDR3 RAM is still a little more expensive than DDR2 right now, and a single 4GB stick costs more than 2x2GB sticks (I'm sure someone will know why), but I think it still shows how overpriced some of their products are.
And SO-DIMM format (the laptop memory, that is) makes it even more expensive. Still, one could get the entire 2 x 4GB 1066 DDR3 SO-DIMM set here for about 500 euros - if Fujitsu-Siemens is good enough name for one. It's probably re-branded memory as well, I doubt they have a plant capable of producing DDR3-memory. Apple most certainly does not have, AFAIK the Apple-memory is made by Kingston. Might very well be the very same memory, even.
The reason why single 4GB stick costs more than 2x2GB sticks is yields, especially if the larger stick uses the same amount of chips, meaning that a chip must be twice the capacity. Increasing the amount of semiconductors on a die pretty much results in lower yields, as the chances of finding a faulty one from twice as many is... well, probably twice as large.
Leading to twice as many non-working chips.
Same is true for processors - especially in the case of models with very large cache, as that often holds the most of the semiconductors in a CPU - and GPUs, which have insane amount of semiconductors. Larger models are probably not that much more expensive to manufacture per unit, but you'll end up throwing away much more of them. This is also the reason why both ATi and nVidia sell graphics cards with more or less GPU pipelines deactivated; these are the chips that were faulty and couldn't be used in the flagship product.
...and the reason why Cell has 8 of the... whatevertheywerethingies, but only 7 are active. One is a spare, so single faulty... thingy does not mean loss of entire Cell.