Its almost impossible for the average punter to compare...

Its almost impossible for the average punter to compare corporate expenses of different companies because of the way they treat those various expenses.  It can be done, of course, but only if you have a few hours to bother doing it.  Even then, you'd need a contact in each company to ensure you got things in the right boxes.

The popular measure is cash cost of production, but even this is clouded by what exactly is included (ie, C1, C2, or C3 cash costs)?

As an example, Atlas' last quarterly announced a C1 cash cost (direct production costs) of $38/wmt and a "full cash cost" of $57/wmt CFR .(http://www.atlasiron.com.au/site/PDF/8302_1/HalfYearAccounts).  What does "full cash cost" mean?  Ostensibly, its the C3 cost, but its not.  Atlas defines its full cash cost as "C1 cash costs, royalties, freight, corporate and administration, exploration and evaluation, interest expense, contractor profit share and sustaining capital expenditure but excludes depreciation and amortisation, one-off restructuring costs, suspension and ramp-up costs of operating mine sites, and other non-cash expenses."  But its ambiguous, but not  C3 as D&A are excluded.  Importantly, it includes their "contractor profit share" which is not a preprofit cost but is included because its a liability!

MGX reports an "all-in group cash cost" of $45/t fob (https://www.mtgibsoniron.com.au/wp-...-MGX-March-Quarter-2018-Activities-Report.pdf).  A footnote describes "all-in" as "FOB and include cash operating costs, royalties, sustaining capital expenditure and corporate costs, excluding Koolan Island restart expenditure."  Again, D&A is excluded.

Good luck trying to meaningfully compare Atlas' $57/wmt costs to MGX' $45/wmt.

At the end of the day, its the declared profit and the ROC that are important.  MGX is making cash.  Atlas is going down the tube.  End of story.