the cynical - it is the sector average computed by Ambrian in a note on the sector last Summer. Certainly has to be taken with 'a dose of salt' as there are many variables specific to individual companies.
But as a quick rule-of-thumb they suggest fair value for exploration companies is Enterprise value/resource oz = £19.6
Fair value for Production companies is Ev/Prod oz = £2,181
I just run a sheet with many of the AIM explorers and producers and look out for wide discrepancies away from the average.
Chip