Crutchfield carries their stuff so that may give you better pricing
http://www.crutchfield.com/sonos

Networked Mac Mini’s in each room are a good approach (but a little pricey) and even less expensive would be networked Apple TV’s. I have an apple tv in my media center connected via hardwired ethernet, but the built-in wireless works as well. It gives me direct access to my iTunes library and it can either pull content from iTunes, or be used as a "speaker" where iTunes pushes content to it (which is the mode I use to feed music to my hardwired whole house audio system). The apple tv can also store content on it’s hard drive as well for local access. The advantage to the Apple TV approach is that it serves video as well so you could be in your master bedroom and watch a movie stored on your computer or rent a movie via the Apple TV connection to the iTunes store right from the bedroom.
Sounds like a fun project.