When trying to assess the value of team members, here are a number of questions that can be easily answered - and when aggregated, will easily indicate the value you assign to them.
If you could "clone" this person, and have another one just the same - would you?
If this person told you they are leaving tomorrow, would you be more anxious - or more relieved?
If you were to built a new team from scratch, and could pick anyone - would you pick this person?
Another question competing for a place on this short list is:
If this person left the team - how critical would it be to replace them? How hard would it be to replace them?