VIA C3, in itx form factor, less than $100 for motherboard and CPU and its bus speed is double what the apple's is.Add a 40gig HD, thats what about $35-$50 these days? Plus a dvd/cdrw drive, and RAM, less than $300 easily.
Want to sacrifice that tiny little form factor for micro atx instead, $300 gets you a 2.4ghz or better machine with a bus speed triple the apple's.
edit:
pricewatch is showing a 2ghz VIA C3 MB combo for $74 shipped.