The WD My Book Thunderbolt Duo 4TB has an average price of £393, the 6TB version costs around £472 and you have to add the £44 for the Thunderbolt cable. It's not cheap, no matter how you look at it. If you also consider that the two 2TB drives that WD used cost £103 apiece, then it's hard to be sympathetic with the price.
The performance isn't exceptional either. Yes, it's faster than USB 2.0, but if you look at the measured transfer rate of the WD My Book Thunderbolt Duo 4TB, 200 MB/s, then a Firewire 800 will do the job just as well. The same is true for a USB 3.0 connection. This external drive is interesting only if you really want a Thunderbolt device or if your computer (read Mac) has no other connectors. On the plus side, it does come with a three-year factory warranty.