There are some cheaper head tracking methods than TrackIR for sure. You can buy a PS3 camera pretty cheap on ebay ($15) but you'll have to remove the IR filter yourself. Use that with a homemade IR LED clip and Opentrack. The TrackIR sensor has a higher res and a faster scan I believe though.
I've read that it's likely unnecessary to remove the PS3 lens filter. Just put a piece of floppy disk or exposed film over the lens. Or buy a filter for $5.