As a generalisation, they (well, the towers anyway) tend to have a longer useful life so are priced accordingly.
A friend of mine (magazine designer) still uses his G4 450 at home for Photoshop & Quark... brings his Carrara Pro files into work for rendering on the dual G5 2.5s however.
My dual 1.42 came off pretty well in the Photoshop test thread that was around here a few months ago... pretty close to a dual 1.8 G5, I think. And that's a Mac that is 3 years old so it's still got another 2-4 years of working life in it yet, I reckon.