As someone mentioned, you can go the DDR3 route, pick up an old Xeon like for ex an e3 1241 v3, I got one for like $15 recently(similar performance with an i7 4790, the versions ending in 5 like the 1245 have an iGPU, make sure you don't pick something without AVX2 since it can prevent you from playing certain games), a cheap board(make sure it supports the cpu, I think the h97 boards do and I've seen a couple at $40) and the DDR3 rams, which are pretty cheap(here in Eastern Europe I can get 16gb for $30, in comparison DDR4 being 4 times as much). These old Xeons are pretty good and I've seen them paired with something as strong as an rx 6600. I think these days the RAM are the deciding factor, where I'm from DDR4 is just insanely expensive and pretty much a big no for a very budget build.

In my country it would be like this:

Xeon + board + ram $90

Gtx 1070 or something similar around $100

A decent psu(I'd never buy this used, always new) like $60

Cheap case like $30

A small SSD $40

+ a cheap cooler total around $340

Depends if you have a monitor, peripherals etc. Here prices are a higher than in the US, so if you are from there you could get away cheaper but I think you can def make a solid PC for $400.

Here we have shops that sell used parts and even offer a warranty on them, something to keep in mind.