Not that expensive. For a couple of hundred dollars you can build very nice 60-90fps IR eye tracking system. You'll need IR camera, IR tele lens (very important), couple of IR sources and open source tracking software. See http://www.gazegroup.org/develop/

There is also alternative approach. If you are just recording and don't need interactivity - all you need, is a regular camera with zoom and a small tripod. You can record on an SD card and synchronize the stream stream time in post-processing. Just flash the monitor screen couple of times at the start and the end of the recording, and play tracking alignment sequence.

Here's an attempt to do just that: http://www.youtube.com/watch?v=zJqKRL9D2qY

I somehow assumed this was done with a webcam eyetracking software .. my bad.