Monitor, mouse, keyboard, and such will drive the cost up a bit. Those parts tend to last longer so used market availability is low.
This is what the cost of an entry level rig would be if you bought everything new.
PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel Core i3-4150 3.5GHz Dual-Core Processor | $109.99 @ SuperBiiz |
| Motherboard | Gigabyte GA-H81M-H Micro ATX LGA1150 Motherboard | $39.99 @ Amazon |
| Memory | Kingston Savage 8GB (1 x 8GB) DDR3-1600 Memory | $31.99 @ Newegg |
| Storage | Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive | $46.89 @ OutletPC |
| Video Card | EVGA GeForce GTX 950 2GB Video Card | $139.99 @ Newegg |
| Case | Fractal Design Core 1100 MicroATX Mini Tower Case | $26.99 @ NCIX US |
| Power Supply | EVGA 500W 80+ Certified ATX Power Supply | $34.99 @ Amazon |
| Monitor | Acer G226HQLBbd 60Hz 21.5" Monitor | $89.99 @ Amazon |
| Keyboard | Cooler Master CM Storm Devastator Gaming Bundle Wired Gaming Keyboard w/Optical Mouse | $27.69 @ Amazon |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total | $548.51 | |
| Generated by PCPartPicker 2016-02-29 22:27 EST-0500 |
Good things to buy used:
Graphics card. Used HD 7870 would perform the same as a GTX 950 for significantly less.
Bundles. Sometimes you can get a good deal on a package with a Core 2 Quad + motherboard + RAM, for example.
Older i7 + good motherboard + 6GB of RAM for $135 shipped. Look for deals like that on places like /r/hardwareswa[/