The Saris Bones RS rack fits on almost anything and I have been able to install mine in less than a minute on every vehicle I've tried. Check their website to see if it will work on your Prius.

Aerodynamic drag force increases by the square of the velocity. The Prius is pretty aerodynamic which helps it achieve it's good mileage at highway speeds (50mph+), and hanging a rack with a bike or two on it will definitely affect your highway mileage. In town, not so much.