Fostex didn't just design the drivers for the Denons. They also designed the OEM housing. Basically, the whole headphone was done by Fostex for Denon.
The D7000 is pretty hard to find new for $700 these days in my experience. Sure, it can be done, but it's not like they go for that on Amazon regularly anymore. Most of the good retailers still have them for $800-$999. Also the $2300 price on the TH900 is AudioCubes' inflated importer price. AudioCubes is always much more expensive than PriceJapan and other deputy services. I wouldn't be surprised to find the TH900 for closer to an even $2000 when it's available on PriceJapan. So I think "more than triple" is a stretch.
Add to that $400 for a nice pair of cups to make your D7000 look as good as the Fostex and you're getting even closer with the price.