Each Resource has its own cargo cost per unit transferred.
Here is a chart showing the cargo cost per unit of the resource
|Resource||Cargo cost per unit|
Cargo costs are multiplied by the average distance between source city, destination city and cargo city.
In the below example, a trade route between the RED cities has a distance of 6. If cargo is provided by either city the cost factor would be 6+6/2=6. If the cargo was provided by the PURPLE city, which has a distance of 3 between both, the cost factor would be 3+3/2=3.