Grade and size not the only complications as you have colour and...

Grade and size not the only complications as you have colour and quality of diamonds also.
Successful mines have ranged from Argyle with high grade close to 1ct per tonne, small diamonds, mostly brown poor quality with some exceptional pinks low price per ct approx $25USD.
To Letseng in Lesotho with a grade around 2ct per hundred tonne but with large type 2 diamonds average over $1000/ct.
Ellendale costs will range significantly as the Lights tailings are just sitting on the pad, the alluvials are relatively shallow, then there is the pits.
If you are really keen you would need to go back through KDC documents (2002-2015) on costings, though I would tend to ignore data in 2014-15.
Reasons for Blina stopping in 2008 are highlighted in the pricing chart you have (GFC).
On another note the chart is tracking polished prices, these don't always align with rough diamond prices (another complication). Rough are harder to track as each company have there own valuation systems and are more opaque.
I would also ignore price spikes of 2008,2011, 2022 as they have never held and are more reflective of an overheating market.
The Ellendale diamonds are unique, especially from E9 at approx 11% Fancy yellow diamonds, most mines produce around 0.01% fancy colours.
Prices are well above world average E9 was in the top 5 prices.