Demo is free, or you can pay $15 for the Alpha which gets you access to whats shown in the video + all future development updates and the final product.
This article gives you a rundown of the game/demo. Check out the video.
http://hooniverse.com/2013/08/12/beamng-drive-tech-demo-and-alpha-now-available-for-all/
I was just playing around on the demo for the past hour. the damage is pretty brutal. also the vehicles are completely configurable. as in, if you want to strip it down to just a motor and a frame and drive around, you can do that. the truck in the demo you can add off road suspension and tires to tackle obstacles.
I am really tempted to buy the alpha now.