I seriously doubt you need to spend more than 100% the cost of the 8 GPUs on the rest of the system.
If your 8 GPUs cost ~6k USD, you should be able to build a system for under ~10k USD (even ~8k). Any extra money you spend is more out of desire to "max out" your specs and less of a performance boost.
If your 8 GPUs cost ~6k USD, you should be able to build a system for under ~10k USD (even ~8k). Any extra money you spend is more out of desire to "max out" your specs and less of a performance boost.