Given your discomfort with the word objective, and also that this thread started because you took issue with the idea that the value of labor is subjective, you believe that, with perfect information, there is some dollar amount that you and the prospective hire can agree is fair compensation for their services?
But at the same time, you acknowledge that as a hiring authority you lack access to that perfect information that would give you that exact dollar value, so you're operating off of the limited information available to you.
The reason why I accused you of being an HR person at a high-turnover corp on that basis is that to suggest that the value of labor isn't "subjective" means that it must be "objective", and I believe that only someone doing hiring at one of these corps could truly believe that they had an objective understanding of the value of a hire going into a salary negotiation.
it can be quantified
by seeing how much value it adds to the product
factoring in also the cost of training
and cost of equipment