I suspect that it's just plain expensive technology to begin with, and since it's proprietary Apple property it's probably extremely over-priced, particularly from non-Apple vendors.
Thunderbolt is
NOT an Apple proprietary product. Like USB it is an Intel product. In fact Thunderbolt 3 and USB C (used on the new MacBook) use the same connector. Apple has always been an early adopter of new technologies and that inevitably has implications for cost and risk. I am confident that by the time PCs have adopted Thunderbolt and prices have come down, Apple will have moved on to still newer technologies. If you want cheap, buy Dell not Apple.
Thanks for the clarification. Yet, since Apple is the
only company that has, so far, adapted it, it will remain expensive. Interesting that no other companies (that I am aware of) adapted Firewire 800, which was expensive for some time. Wonder if Thunderbolt will follow the same pattern?