It would be cool if Intel spoke out about the actual cost of Light Peak -- I mean, _can_ it be as cheap as USB 3.0, or is it inherently an expensive technology?
FireWire controllers, for example, are still many times more expensive than the USB equivalent.
It's going to depend what you do with it. Thunderbolt is basically an external PCI-E, with some additional brains to handle chaining and the DisplayPort signal. I'd say it's always going to be more expensive than USB3.
Not necessarily by much, but with USB the vast majority of the work is really done in software and on the machine's hardware, which pretty directly controls the USB device. That's, in fact, why FireWire was so much superior to USB early on and could reach much higher data rates without straining the machines: the dedicated hardware meant almost no resources (outside of providing or consuming data) were required of the machine (I remember machines in the late 90s and early 00s having trouble playing MP3s in Winamp and driving a USB communication at the same time)