James is right... it is difficult to find statistics that allow you to make the kind of 'apples to apples' comparison that you are looking for.
In addition to help desk strategy, consider the other influencing factors within the environment. Standardized desktop hardware versus 'deal of the month' approach, standardized desktop configuration versus custom installed, implementation & use of automated management tools, enterprise packaged applications versus custom developed niche applications, etc...
There is no shortage of statistics. It has been my experience that any statistics that you do find will have been carefully calculated to support a particular perspective (usually a vendor's) and do not include all of the context to allow for an accurate comparison.
In a previous job I had looked at the ratio of IT spending to revenue, but found it to be at such a high level, it only confirmed what I already intuitively knew (as do you) - the organization was running 'lean'.
It is a multi-dimensional problem, with a virtually infinite number of permutations and combinations. Kind of like.... a fruit salad.
Sorry Colin... not much help here. But if it is any consolation, you are not alone.