Logic boards and CPUs with the necessary PCIe lanes capable of feeding the GPUs, storage fast enough to do the 30GB/s that is in the spec sheet, power supplies

Logic boards and CPUs with the necessary PCIe lanes capable of feeding the GPUs, storage fast enough to do the 30GB/s that is in the spec sheet, power supplies and a case to hold such a power hungry system, etc. is not cheap. $15k seems like a kind of low - if you've tried to spec out a server with similar quantities of accelerators recently, you'd have trouble hitting that figure, even if using consumer grade GPUs.