$300 for a TB bridge with crippled USB3 hub ("2Gbps max"), FireWire, audio and GbE... that's maybe $40 worth of parts including the external power brick. Not really worth more than $100 to most people and even that may feel like way too much for so little to many. For that sort of premium, I would have expected it to also have at least one internal bay for a 2.5" HDD/SSD.
Of course, this sort of thing has niche-market status at this point in time due to how few computers have TB ports, how much of a premium most products with TB carry and how even fewer may actually use them, therefore rather low unit count expectations to spread development costs across.