It's a difficult calculation and you can only get averages.
Some freight is carried as and when space is available, and attracts a lower rate, whereas other types, for example urgent and perishable, are booked and must travel on a specific flight, thus attracting a higher rate.
There is also the question of different commodities attracting different rates based on the difficulty of handling and the volume/density. As an example, 200 kilos of books would be easy to handle, non fragile and possibly non-urgent, whereas 200 kilos of flowers would be urgent, perishable, and low density and would thus need to be charged a much higher rate.
When I worked on the freight side there were three different rates, I think it was overnight, 2 day, and general freight and that we worked on a very rough average figure of USD 1.00/0.50/0.35 respectively per kilo/1000 km.