Now is the time - Copper back to June 2018 prices - $7000 per tonne, page-339

I like to use an Analogy that is easy to understand... value per tonne

convert the Value per tonne

let's say copper is $7000 per tonne

10% = $700 per tonne of rock
1% = $70 per tonne of rock
0.1% = $7 per tonne of rock

same can be done with gold

approx $1800 per ounce (Oz)

3.2 grams is 10% of an Oz
3.2 grams is worth $180 per tonne

or $180 per Tonne of Rock

I don't get overly excited til the $ per tonne is more than the price per tonne of my garden mulch.

try doing the above with Iron Ore and you will see why they are doing ok right now.

under $100 per tonne is a tough gig but not impossible, under $75 per tonne is probably uneconomic for an open cut,

Underground is trickier again depending on CapEx costs.