The PIAA horns are great (under $50) and an easy install. Check out this thread. http://priuschat.com/forums/prius-modifications/25402-piaa-sports-horn.html
I'm considering a Stebel Nautilus for my bike (nothing like a motorcycle with a horn that sounds like a freight train derailing in the immediate vicinity!)... maybe I'll get one for the car, too. The instructions caution not to blow the horn in your garage... it can damage your hearing.