I've recently discovered a cheaper alternative named EdTracker. One can buy either a ready-to-ship version (EdTracker Pro) which costs £45.00 (~ $68.5, €63.5), or, if you are not afraid of some soldering and hand-work, a do-it-yourself version which is even cheaper.
I've got myself a Pro version and tried it, and I can tell it works quite well. Unfortunately, I don't have TrackIR so I cannot compare them directly, but from what I've heard and saw in videos, the comparison table looks like this:
Cons:
* EdTracker only supports 3-axis turns, but not lateral movements. That is, in ETS you will be able to turn around however you like, but not move closer to, or further from the dashboard.
* It must be mounted directly on top of the headphones, no side-mounting is allowed.
* No clip is supplied, you'll have to devise something yourself (double-sided sticky-tape or something like that).
Pros:
* It's cheaper.
* It's smaller (a small box instead of two long "horns").
* It works by means of accelerometer, so it does not require a separate movements tracker on a desktop.
* As a consequence, no problems with blocking the light dots from the tracker's view.
From personal experience I can tell that I quite liked it. The lag is present, but it's very small (about 0,1–0,2 seconds), so if I don't pay special attention I don't even notice it. After playing it for a while I watched Sunday Night Trucking and caught myself moving my head trying to move the Squirrel's in-cab camera and watch sideways.
