I think there's a blend of art and science to this. It is much the same as judging value in an employee for a given job. In my line of work, I use the terms 'starters' and 'bench players' a lot. But back to your question:
A player's value is judged by many factors. I think most would agree on the factors; it is a matter of prioritizing and ranking them when making a decision on a player. Athleticism, experience, pure basketball skills like shooting/rebounding/dribbling/passing, etc., toughness, team approach, coachability, commitment, etc.
I'd like to see how Geoff Petrie would answer this one....