Well, you can obviously get cheaper computers, but if you want something powerful that will run all applications for awhile you've gotta cough up more cash than it takes to get a low level pc :)
Linux is great for computer savvy people, but comparatively speaking, there is so little compatible software & drivers vs what is available for windows. What if he wants to add a tv tuner card later, or decides he wants to play that video game that came out a little while ago? He'll have to do additional research for drivers for that particular linux distribution (not always an easy task as I'm sure you well know).
Personally I figure that the people who actively search out linux for themselves and figure it out on their own how to use it are probably the ONLY ones who should use it. Otherwise it's just problems waiting to happen.