But the Drobbo Firewire 800 equivalent DOES offer BeyondRAID and a quick google search reveals the enormous premium they charge when the Firewire 800 version is £299 compared to £629 for the Thunderbolt version. NOTHING can justify such a large markup.
Are you comparing the 5D to the current-model DR04DD10 (the 4-bay FW800 Drobo)?
If so, while the DR04DD is less than half the price, that's really not a very good comparison; leaving TB interface aside entirely, the 5D has 5 bays over four, and has an MSATA caching SSD slot available which, if it works, pushes it into a substantially higher tier of storage.
Now that the 5N (the NAS version of the 5D) has been announced, we can say more or less exactly what the price premium for Thunderbolt is: $250. The 5N, which is identical so far as I can tell other than having a NAS interface instead of TB/USB3, lists for $599, the 5D for $849.
You're free to argue whether that's an early adopter cost ding, whether it's because it actually can push data faster than the NAS version when fully provisioned with an SSD and 5 HDs, or whether it's just overpriced, but we're not comparing $350 to $850, and as I pointed out before, you would be very hard pressed to find any other RAID solution that includes 5 drive bays, live provisioning, and a caching SSD for a lot less than that.