Also still using some parts of my older computer. First purchase was the CPU, it was a great deal that triggered the rest of the build.
GPU: $50, though mine is a weaker Radeon 6750, I'm not a big gamer
Motherboard: $100 on an Asrock Z77 Extreme
RAM: 8GB came bundled with MB
CPU: $100 for an unlocked i2500k
PSU: $50 for a 600W Bronze
Case: $30 for a Xigmatek Asgard
Harddrive: Using my old HDD
SSD drive: $70 for a Samsung 830 128GB
DVD drive: $30 for a Blu-Ray recorder
Mouse: Using my old mouse
Keyboard: $20 for a more ergonomic keyboard
Windows OS: $30 for Win8 upgrade with my old copy of XP
24 inch LED monitor: Using my old 24" LCD