I am afraid it would be difficult to find a mechanism which would determine those factors correctly. The only possible factor I can imagine about this optional system would be some purge “kill count” for the player himself.
But according to the new purge being intended more as a tactical and strategy thing overall (meaning counting with the usage of the thralls, cauldrons, forts, etc., and not so much about self-challenged experienced players cheesing the purge by themselves any way possible), it possibly won’t be even considered to be implemented into the game.
In my opinion making the effort to create functional defense using thralls and other possibilities is not something “less worthy” than one player dealing with the purge without using anything except his force and personal battle tactics.
So I see no reason to give a player who is dealing with the purge the way it was mentioned lesser reward than to someone who just needs some serious overkill challenge. 