A GTX 750 Ti will get you in the door for about $150 or less. It's been said to be a step above the Xbox One and even outperforms the PS4 in some cases. That probably won't hold in the future as devs continue to optimize both consoles. But it's an efficient card too meaning you won't need a crazy power supply or extra cooling.

But Spuuky's right in that you will have to cut corners, namely a cheap motherboard, case, CPU, its stock cooler, and not a lot of ram. A monitor probably brings you over the console price if you're not recycling.

You can actually get a legit key for any Windows version you want for as low as $15 from the Microsoft Software Swap on Reddit. They got Office and all kinds of stuff, too, it's awesome