a camera (must be bought anyway)
...A camera with removed IR filter to be precise.
Not everybody will consider himself capable of removing the IR filter of the camera which he "bought anyway", and definitely not everybody will be happy with the impact on normal webcam operation.
https://www.trackhat.org/shop-cfk4
Trackhat is a commercial competitor product to TrackIR and that site only offers products by Trackhat.
It's okay to link it, but promoting it as a site that lists "the full range of options" is a bit misleading.
As I said: You get what you pay for.
If you buy TrackIR 5, you've cut it all.
It you buy Trackhat, you've cut most of it.
If you buy something from 3rd party source (Amazon, ebay, whatever), you probably cut most of it, but you rely on the quality of someone you don't know.
If you decide to try a DIY solution, you save money in exchange for your own contribution - it depends on your abilities then.
Mike