Most people tend to forget that if they build their own machine, they don't pay for the cost of making it!
If Alienware didn't charge for the cost of hand-building, testing, burn-in, benchmarking, custom setup and installation, tweaking, feature-matchup, etc, etc...then they could charge you only 2500 bucks.
If you put a jaguar on a fully-automated assembly line, with no customer care or support, you could cut the costs in half (at least!).
The bottom line is: If you want a machine built FOR-YOU by a world-recognized company, Alienware is at the top of the list. If you want to build it yourself, then Alienware is off the charts.