I wrote a UX tracking tool for tablets[0]. It ended up not going to market simply because you're right: processing power is not cheap enough. I started with on-board processing, but that led to substandard result. We then moved on to cloud based processing, but having cloudbased processing would increase costs a lot and we couldn't keep the pricepoint.

Lastly, such things are difficult to market and we weren't ready financially to take on such risks.

[0] - http://blog.chewxy.com/2014/04/08/eyetracking-jetpack-joyrid...