The Drobo Mini comes with both Thunderbolt and USB cables included.
You're not just paying for a HDD enclosure, but also the Drobo 'redundancy' with regards to hot-swapping HDD's that fail.
Its priced in the 4bay NAS category but its not really a NAS, so I'd say its more expensive, given what you're getting. Its an interesting product but drobo's reputation and price make it something that I'll not buy.
But considering you can pick up a 6TB Synology NAS for only a little more, adding all the full on NAS features, it's still way overpriced for what it is.
Remember, to fill it with 4 1TB HDDs you are looking at around an additional £400. That makes it a REALLY expensive option.