So … you are saying the markup is anything but large. It’s a tiny markup, especially considering the case you get. (I wonder what noise your built PC will make.)
Yep. $400 on a $5000 machine? For me, that'd easily be covered by the case, acoustics, and ability to use OSX. Not to mention the knowledge that I've got warranty on the full machine, not just on each individual part. My custom-built desktop is fine for gaming, but if I'm dropping $5k for a professional machine, I want some guarantees.