what config for 3000-4000$? because if you buy older used servers you can have 10 or more servers for this price 
of course they will not be super energy efficient like ryzen or so, but... who cares if you have super cheap electricity... (10k bill /year for 100 computers (300k /3k))
Briefly, I mean a PC based on latest available CPU like Ryzen Threadripper 3970X, at least 64 Gb of RAM, 2-3 powerful GPUs like RTX 2080Ti (it's a subject of research which GPU to use, but for now it looks that 2080Ti is the best for most GPU demanding project, GPUGRID) and a 512Gb NVME (larger size not required). Sure, a case, a PSU and a motherboard are also required. But I see no reason to select specific components right now. of course they will not be super energy efficient like ryzen or so, but... who cares if you have super cheap electricity... (10k bill /year for 100 computers (300k /3k))
