A benchmark ball is any ball that you feel you can read the lanes best with. Whether that means something in the middle, or at the ends of the spectrum, it doesn't matter. In other fields, benchmarks are usually at the top, so not sure why bowling balls would be in the middle.
Someone mentioned the Cash being a poor benchmark because there would be nowhere up from there. This would be true regardless if it was a benchmark or not. As long as someone is able to differentiate between not hooking due to oil and not hooking due to roll out, there's no reason the Cash can't be a benchmark. Either way, if you need more ball, it doesn't exist.
Pick any ball that you know well enough that you can decide if you need to throw something else or not. If you are just starting out, then middle of the road would be a good starting point.