You're going to need some way to 'see' the area around the car, and to track the speed of nearby objects. Google uses a combination of LIDAR, radar, conventional cameras, and occasionally sonar (see here for a high-level overview). This technology is quite expensive, and can easily cost thousands of US dollars.
However, a bigger obstacle than the expense of the hardware (which would be smaller for a table-top prototype) is the software complexity. Like many major projects, the software for self-driving cars is the result of years of work from AI research teams, and thus extremely difficult to duplicate on your own.
That said, you're not trying to make a state-of-the-art self-driving car. Assuming you're an expert in image processing and robotics, you can probably create a basic prototype, (like something that drive in a limited table-top environment). However, it's still going to take a lot of time and money.