The only option that fits your budget today I can think of would be picking up one of the old xeon combos off of AliExpress. I spent like $100 on a MB+CPU+64GB DDR4 combo with a 2880 v4 I think. 14c/28t at any rate. You can probably grab a case/power supply/video card used for under $50 on eBay.
Please note that I’m not saying that this is a good option; it took a lot of fiddling for me to get mine running smoothly. But if you’ve got more time and patience than money, it might work for you.