$100/hr is considerably more than I make as a software engineer. if my employer was required to pay a janitor $100/hr, they would fire them and have all the junior engineers spend the last hour of their day cleaning. having someone clean the building is essential for any business with a physical presence, but it's only worth hiring a person specifically to clean if they can be paid less than your core employees or if they can do it much more efficiently. this is why restaurants typically do not employ janitors.
Another example that has come to mind lately is how do you quantify non-production skilled or intellectual labor?
The value of maintenance, safety, quality are often only only apparent after being cut
They may be absolutely critical to the business running even though they don't produce revenue directly.
They don't produce revenue, but one must assume they provide some value and in this case avoiding the loss of revenue due to theft, fires, etc.
When someone is directly contributing to revenue, like a factory worker or salesperson, it's a bit simpler since you can look at their output, but you still need more information than gross margin to know the maximum a company could profitably pay an employee, since a company might design their operations very differently depending on the price of labor.