TH-600: I think $500 is a fair price.
TH-900: It's hard, because I think it's very good at a certain sound signature. It's a very high quality product, but I'm not sure that it's product that anyone would want for the price they're asking. It's like if Ferrari made a minivan. Sure it's quality, and expensive and a Ferrari, but what's the point? I'd be willing to pay ~$800 for it.