~ $300 for stereo video on PI.

Dual Camera input PI $215 

http://www.newark.com/raspberry-pi/rpi-compute-dev-kit/raspberry-pi-compute-dev-board/dp/62X6560?ost=62X6560&COM=rpimain-topban-compmod

Camera Boards are $30 each. 

http://www.adafruit.com/products/1367

I think this price is mostly driven by OpenCV users and other hobbiests driving up prices to be too high to use internal to all but high ticket products.