I've previously shown you how to build a Windows 10 PC for under $500, but is it possible to shave another $100 off the build? You bet!
It's actually possible to build a pretty decent PC for under $400.
Could you buy cheaper from an OEM? Sure you could, but I doubt that you could buy much better for the money. Also, because you know everything that went into the system, you can diagnose problems or carry out upgrades much easier than you could on a big-box OEM PC.
If you wanted to give this system more grunt for gaming or graphics processing then you can easily add a separate graphics card. Something like the ASUS Radeon R7 250X, which retails for around $120, would be a good start, and would still leave you with a pretty cheap, yet quality, system.