Anders,
Shop around and let me know what you find. There's a locally owned tire business near my parents called Bubnash Tire that is generally lower than anywhere in price, including mass-volume internet sellers like Tire Rack. They don't charge for installation, valve stems, etc. The price you are given is the price you pay, sans tax of course. Even when I was a manager at Pep Boys, I bought my tires from Bubnash. I've recommended the place to several friends, most of whom ended up buying their tires there because the prices are usually unbeatable when you factor installation and shop fees from other places.
I'm actually getting Kuhmo 4X's installed on my WRX there tomorrow. If you tell me what size tires are on your Prius, I'll get you a price while I'm there. It'd be a small hike for you (about an hour each way), but, the price may make it worthwhile.