Depends on the renderer they use but gpgpu, opencl, cuda and the like are just not as wide in capabilities as a cpu can do. Most modern places use combined cpu and gpu, like how pixar and dreamworks have for decades or even supercomputers. Gpus are much more efficient with parallel computing. But $1k will get a better single pc than multiple. If you need a render farm, you are probably better off using a render farm service with a low budget.