This all depends on your local prices and exactly what you need to buy. Windows license, Monitor/Keyboard/Mouse will be $250 right there assuming you go for a lower quality keyboard. A good gaming keyboard and mouse will be over $100 just for those if you go with mechanical and a cheaper 22" or so monitor is at least $100.
A lower spec gaming system would be an i3-3100 CPU (or wait till the new CPUs from AMD and Intel are available on the market), 1151 socket motherboard, about $60, 8 GB of RAM, 256 GB SSD if you keep to about 4 games installed, 512 GB drive if you want to have more games on the system at one time.
You can probably save some money by getting a used monitor, that is less risky to buy than a used CPU or video card.