I think before you say that P/cash at 14 isn't cheap, I would really like to know what is cheap. That way, for all companies which have net debt, their market price is astronomical, as per your contention. There is the odd case where a company may have high cash relative to its marketcap (smartlink, Hindustan Zinc, NMDC), but that is hardly ever the case. Indeed, what is the point of holding large amounts of cash @8%, rather than returning it to shareholders.
I really don't think you should use such a metric.