You might be seeing a premium price for discontinued hardware. The current lineup of WD Green 2TB drives are ~$100. There is no way for me to know if these are lower quality than the previous model or not.
Good point. I'm curious as to which HDD models Apple uses nowadays for their Time Capsule, that would allow for a better comparison. I couldn't find that information anywhere.
By the way, my first generation Time Capsule has a 500GB Seagate Barracuda ES, which used to retail for over $300.
That's how much the hard drive is now. That teardown was done before the Thailand floods, which I'd imagine are related to the current Amazon price. [1] The WD20EARS was only $100 well before that Time Capsule was released. [2]
That drive is listed as $169 MSRP at Amazon. [1]
[0] http://www.engadget.com/2011/06/25/updated-time-capsule-open...
[1] http://www.amazon.com/Western-Digital-Caviar-Desktop-WD20EAR...