Driverless cars don't have to be custom-built: you can add a kit to an existing car that contains a Linux box, LIDAR, cameras, and appropriate machinery to drive the car's controls. Most of the existing Google prototypes have been Priuses or Lexuses with the kit. IIRC, all the components are pretty inexpensive; by far the biggest cost is the LIDAR, but that's because they're not produced in volume and so you have to pay custom-built prices.
