What technologies are needed for a self-driving car? Google, Tesla or Apple have all built or are building their own self-driving cars. As an expert in a related area, I am interested in knowing at a high level, the systems and techniques that go into self-driving cars. 

How easy is it for me to make a tabletop prototype (large enough to accommodate the needed computing power needs)?
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.