There isn't one, because all cores share a single bus in and out of the CPU, and only one of them can be sending/receiving data at a time. So it's kind of a meaningless figure to calculate.
I am looking into making a custom rig...and I am trying to pick out a processor.
So what exactly is bus/core ratio....what is an ideal bus/core ratio?
Does this matter when purchasing a cpu...or should I just ignore it?