Well u happened to pick the most expensive computer, and ofcourse there the extra price is the highest.
But for example this:
AMD Athlonâ„¢ 64 FX-55 with HyperTransport Technology
Microsoft® Windows® XP Professional with Service Pack 2
Alienware® 460 Watt Max Power Supply
alienware case
Alienware® motherboard based on VIA K8T800 Pro chipset
NVIDIA® GeForce™ 6800 GT 256MB DDR3
2GB Dual Channel Low-Latency DDR PC-3200 at 400MHz - 2 x 1024MB
250GB Serial ATA 7,200 RPM w/8MB Cache
16x Dual Layer DVD±R/W Drive
Integrated 5.1 High-Performance Audio with Digital Out
3.5" 1.44 MB Floppy Disk Drive
Integrated High Performance Gigabit Ethernet
All that for $2,653.00
And that is a damn fine pc, and yes it's going to be cheaper if u build it by yourself, but the difference in a pc like this is nothing like $2000.