My guess on his prices:
GPU: $180
Motherboard: $100
RAM: $50
CPU: $200
PSU: $80
Case: $60
Harddrive: $150
SSD drive: $120
DVD drive: $30
Mouse: $30
Keyboard: $20
Windows OS: $120
24 inch LED monitor: $160
Total: $1200-ish
Branded PCs seem cheaper than that because they're rarely sold with a monitor anymore. And most don't have an SSD drive either. So they end up priced at $899 with crappy internals.