That actually not the best metric to use, it's too simplistic, unless you want to normalize the data for different companies in different stages of exploration and resource definition drilling and use weighted averages that account for drill spacing and category of mineralization inferred, indicated or measured resources. I agree that using the same cut-off, and with the category of resource clearly stated, i.e. inferred, indicated and measured, a comparison on an $ Market Cap per Tonnes of target resource is fine if allowing for different mining methods and stripping ratios.
At the start - all exploration is 'inefficient" because they all drill a lot of metres and haven't discovered the resource yet (no value uplift), then if drilling succeeds there is often a phase where a few drill holes add a lot of potential tonnage (maximum value uplift/excitement) and volume to an incompletely drilled resource, then that "drill efficiency" falls/trails off again as the company drills more infill holes and drills a lot of holes at the edges of the resource to close it off in many directions, and to bring the resource up in confidence from inferred to indicated resources, and then to a measured reserve. This might be "inefficient" in your point of view but its totally necessary and a non-negotiable when it comes to all the stages of resource definition and then pre-mining economic studies. Designing a mine and infrastructure around an incomplete knowledge of where the orebody is, is even more inefficient, have seen too many expensive mills/airports/tailings dams plonked on top of valuable ore bodies in my time.
The argument also falls over with deep but extremely high-grade orebodies such as the Witwatersrand or say Fosterville or Oak Dam in South Australia which would fail the "drilling efficiency" metric because of the high number of metres drilled to prove up the orebody, as opposed to say a shitty low-grade orebody that outcrops close to surface, which could pass the "drilling efficiency" metric, but may never get developed.
Also does not account for other external costs and other external factors, you can drill the highest grade and shallow Cu orebodies in the world in the DRC for $35 per metre, but how does your "drilling efficiency" look when the government takes your mining lease off you, to give to their Chinese mates?
No company drills metres for fun, the drilling done is mostly to find new orebodies, add to tonnes or increase the confidence in tonnes already known and then finally as sterilization/condemnation drilling.....
The cost efficiency of the drilling has got nothing to do with how narrow the orebody is, mother nature sets that irrespective of what kind of drilling you use, and how much you pay for it, and if parts of the resource are below minimum mining width, then you still need to have the drilling to prove it, and with stacked/multiple resources often drill holes will attribute data to more than one orezone. Multiple stacked geometries of orebodies might have a high drill cost efficency but tabular or deep-seated lenses underneath barren cover will never have great "drill cost efficiency" but could very well be economically viable.
Even on a like-for-like basis, say when comparing two fictional WA Spod Li explorers and developers, one company could have an efficient fast but expensive drill contractor and get a resource drilled out faster and more reliably at a high cost, whereas another company could have the cheapest but worst most unreliable drill contractor spraying holes in all directions and getting rods stuck and rig breakdowns so there is an element of time that needs to come into it as well.
(20min delay)