What you’d want to do is take the unit buy price and divide it by the unit volume of the ore. Kernite is 1.2 m³, so 810 Z ÷ 1.2 m³ = 675 Z/m³.
That will tell you how valuable an ore is. You can only fit so much volume in your ore hold, so one ore might fit 312 units, and another might fit 16,666, but that doesn’t mean the ore that fits 16,666 units is more valuable than the other.
To judge them equally, look at the Z/m³, and you’ll find which ores are more valuable. Go by the buy price, just because that’s more likely what your customers are willing to pay.