There is a published IATA tarriff for freight which I venture is very raerly used.
Then you have a tarriff from the airline themselves, generally receved n a monthly or quarterly basis.
Then you have 'specials' on particular routes.
Finally and the most common way of obtaining a freight re now is when you reach a certain point (shall we say 500 kilos) but can be less, you pick up the phone and say Mr. Airline I have x amount of kilos to go to x destination what rate will you give me.
So unless you are in revenue management at the airline I would venture it is quite difficult even for me as a freight forwarder to estaimate what the average would be on a given route. I could guestimate but that is all it would be.
Finally of course as been mentioned above for perishables and hazardous materials you would find yourself paying a premium but still probably not the published rates.
The actual truth of the matter is that nowadays the surcharges usually tally up to more than the freight rate.