If you buy used go to hardware swap. You should be able to get something like a 2500k for a little more than 100 and same for a gpu like a 7970.
Heres a quick list I made
PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel Core i5-2500K 3.3GHz Quad-Core Processor | $100.00 |
| Motherboard | Asus P8Z77-M Micro ATX LGA1155 Motherboard | $50.00 |
| Memory | Mushkin ECO2 8GB (2 x 4GB) DDR3-1600 Memory | $30.98 @ Newegg |
| Storage | Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive | $47.99 @ Micro Center |
| Video Card | Sapphire Radeon HD 7970 3GB Video Card | $100.00 |
| Case | Fractal Design Core 1000 USB 3.0 MicroATX Mid Tower Case | $34.99 @ Micro Center |
| Power Supply | EVGA 500W 80+ Bronze Certified ATX Power Supply | $34.99 @ Newegg |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total | $398.95 | |
| Generated by PCPartPicker 2016-04-23 09:21 EDT-0400 |
This is just to give you an idea. You don't have to buy these exact parts. It isn't really worth it to buy hdds, ram, or psus second hand.