Cost of green beans is a complex topic - part science and part finger in the air.
There can be a relationship between quality and price up to a certain level, then speculation kicks in and it's anyone's guess.
Think of a game where you try and anticipate input costs, reference points (C-index), transportation and warehousing, financing, insurance, risk, market dynamics, hedging, futures and then finger in the air by way of what the market will tolerate in terms of price...........and,,,,,,,,,,,,,,,,,,, then overlay the prevailing business conditions of the seller (their market and volume margin objectives), the competitive landscape, etc. I could go into further analysis but it is rather pointless actually.
Are Sumatrans worth a 25-30% premium on Colombians ? Only if you want to purchase Sumatrans because demand outstrips supply (apparently, but I always seem to be able to find a nice quality Sumatran to purchase).
Yesterday, I took delivery of some expensive Brazils. They are in the higher scale of price and certainly quality. But are they worth 25 - 30% more than the already high quality Brazil's I had in my warehouse ? I had 2x 84 point Brazils side by side, yet one was $2 a kilo more expensive, or $2000 a pallet as I buy them by the pallet - well, in fact it's more than that because you lose 16% with shrinkage.
The seller told me confidentially they cannot even buy coffee from the farmer for the price I pay for it delivered into my warehouse from another broker.
The spectrum is too wide to develop rules unfortunately.
Putting aside any supply issues from origin, assuming normal or standard crop yields, then my sense is that coffees up to maybe 82 - 84 points have some relationship between price and quality. Once you get above that level, you place your fate into the hands of the seller.
Just like luxury cars - try having a comparative discussion with dealers above various marquees and they laugh at you.
Oh, and another point...........points are points are points.........yes, you would hope that Q-graders are on their game !