The parts are typically a lesser quality for those pre-built machines. Here is the ibuypower pc if you built it yourself (or close enough).
PCPartPicker part list:
http://pcpartpicker.com/p/YKwZzy
Price breakdown by merchant: http://pcpartpicker.com/p/YKwZzy/by_merchant/
CPU: Intel Core i7-4770 3.4GHz Quad-Core Processor ($294.99 @ NCIX US)
Motherboard: ASRock H81 Pro BTC ATX LGA1150 Motherboard ($44.00 @ Newegg)
Memory: Kingston Blu 16GB (2 x 8GB) DDR3-1333 Memory ($138.99 @ Newegg)
Storage: Toshiba 2TB 3.5" 7200RPM Internal Hard Drive ($74.99 @ Micro Center)
Video Card: EVGA GeForce GTX 650 2GB Video Card ($99.99 @ Newegg)
Case: Diablotek DIAMOND ATX Mid Tower Case w/400W Power Supply ($29.99 @ Micro Center)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Total: $787.92
Prices include shipping, taxes, and discounts when available
See? For less than $800 including the operating system, you can have the same thing, and that is without going super cheap. Or you could go for something in the same price range like this:
PCPartPicker part list:
http://pcpartpicker.com/p/4yh84D
Price breakdown by merchant: http://pcpartpicker.com/p/4yh84D/by_merchant/
CPU: Intel Core i5-3470 3.2GHz Quad-Core Processor ($189.18 @ SuperBiiz)
Motherboard: ASRock B75M-DGS R2.0 Micro ATX LGA1155 Motherboard ($52.89 @ Amazon)
Memory: G.Skill Ares Series 16GB (2 x 8GB) DDR3-1866 Memory ($139.50 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($51.99 @ Amazon)
Video Card: EVGA GeForce GTX 770 2GB Dual Superclocked ACX Video Card ($329.99 @ Amazon)
Case: Rosewill Line Glow ATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: Corsair CX 600W 80+ Bronze Certified Semi-Modular ATX Power Supply ($49.99 @ Newegg)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Total: $958.50
Prices include shipping, taxes, and discounts when available