For example, for fast 3D renders of small scenes you just care about processing speed, large scenes need good memory bandwidth too, while with LLM training you'd care more about available VRAM, and so on.
But, just to throw out a number out there, you could probably get away with spending as little as 300-400 USD for just a motherboard, PSU, CPU, cooler and RAM, assuming you don't want things like storage, cases and whatever accessories. Basically use it as a server.