Lloyd Shapley already figured out a "fair" way to do this, called the Shapley value.

Basically it's a thought experiment in which all team members of a project (or group, etc) estimate the impact when losing various members of the team (whose work presumably is done by other team members or contracts) on the final cost.

Run some math on the data and you have a group estimation of the relative contribution of all members.

I have a theory that the reason more managers don't use this available tool is that it would probably rate management contribution on the lower end...