Google "freetrack" if you find TrackIR too expensive; you can get almost the same amount of functionality for a fraction of the cost with a bit of DIY skills - nothing too complicated. I used to have such a setup myself, before I lucked out and got a chance to get my hands on TrackIR 5 Pro for cheap.
You need two things, one is a webcam that's easy to modify and not too expensive. What you need to do with it is remove the IR filter and add a visual spectrum filter, essentially making the cam only sensitive to the IR spectrum so regular light doesn't confuse the tracking. For a filter that blocks visible light, you can take apart a 3.25" floppy and cut out a piece of the magnetic foil, and just mount that somewhere between the camera lens and it's sensor.
The second piece of equipment is (usually) a three IR led setup, usually powered by a battery and fastened to either a baseball cap or a headset. That's it for the hardware - the software side of things is covered by Freetrack, which is free. If that all sounds complicated, it's not - and if you're not a DIY sort of guy, if you have any friends that are this would probably take somewhere between 30 minutes and one hour of their time.
Anyway, here's a link to the site with all the relevant information:
http://www.free-track.net
