One of the lightest 18" wheels you can get (at reasonable cost) is the Enkei RP-F1, weighing just 17 pounds for the 18" x 7.5" combo that would probably suite a Prius. I don't know if they're available drilled with your car's 5x100 bolt pattern, but Enkei makes a whole variety of fitments, so I'd assume you'd be able to get your size.
Sample pic on a Miata:
http://www.good-win-racing.com/mazda.../MX5ChrisB.jpg