When Apple debuted its Thunderbolt cable, I imagine a lot of people said the same thing I did: "Fifty bucks? For a cable?!" However, iFixit did a teardown on Apple's Thunderbolt cable and found that it's not just a length of copper wire with connectors at either end. It turns out the Thunderbolt cable comes with some embedded chips designed to manage the high data transfer rate made possible by the Light Peak tech underlying Thunderbolt, and those chips are probably the main factor behind the cable's relatively high cost.
Ars Technica has an in-depth overview of the tech behind this cable and why these chips are necessary for the whole Thunderbolt thing to work. Ars also notes that the tech is eerily reminiscent of Firewire in that it has a high cost barrier compared to USB, applications limited to high-speed storage and video, and is a protocol that has yet to see widespread adoption outside of Apple and a proprietary variation from Sony. When they put it that way... yeah, it does kind of sound like Firewire all over again.