For a more common off the shelf computer, try a full Apple Macbook Pro, comes in at $1,199.
The most expensive Macbook Pro is $2,499.
Mac ProPart Number: Z0EM
Two 3.2GHz Quad-Core Intel Xeon (8-core)
32GB (8 x 4GB)
Mac Pro RAID Card
300GB 15,000-rpm SAS
300GB 15,000-rpm SAS
300GB 15,000-rpm SAS
300GB 15,000-rpm SAS
NVIDIA Quadro FX 5600 1.5GB (Stereo 3D, two dual-link DVI)
Two 16x SuperDrives
AirPort Extreme card (Wi-Fi)
Quad Channel 4Gb Fibre Channel PCI Express Card
Apple USB Modem
Apple wireless Mighty Mouse
Apple Wireless Keyboard (English) + User's Guide
iWork '08 preinstalled
Final Cut Express preinstalled
Aperture preinstalled
Logic Express preinstalled
Accessory kit
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Each company offers its own array of expensive computers.
Dell now owns Alienware who has some of the most expensive and fastest gaming computers on the market.
Apple has the Macintosh desktop which can cost in the tens of thousands but isn't necessarily the fastest machine in the world.
HP offers the Voodoo PC line.
Custom built computers can be faster than nearly any computer you buy and cost you half the price, if you know what you are doing.