Thats the problem, they are expensive. Even a small light laptop that is just a few lbs can have a force of several hundred lbs in an accident or even a sharp turn or panic stop so those stands have to be strong. You can get small plastic desks that sit on the seat and are held in by the seatbelt for under $100.
I worked at a small pc shop that sold winbook/jetbook laptops (don't ever buy one, they are crap) but a small town police department bought a few and mounted them in the cars and we had one that went out the window when they took a turn in a high speed chase. There wasn't a single part on that laptop that I could re-use, screen was cracked, all plastic was cracked/broken, the heatsink was embedded in the keyboard, mainboard was cracked, etc.