Can someone explain theese trade differences?

[PseRamesses]

Could be wrong but I think each faction has a different base value for each resource on the map. It is in a text file somewhere.

Hmm, that would explain a lot. Can anyone point me in the right direction?

[Multiplicity]

There are two dependencies. The first being is the distance factor which influences the value of a trade value. However, it is the

Spoiler Alert, click show to read: 

closest

instance of that resource to your capitol, not just that single resource. The second factor is how many instances of the same type is in a region. For example, I noticed that one region that only has one resource of the same type generates so much per turn. If another reason has two of the same type, it'll produce twice as much. One reason why a place like Timbaktu produces more florins per turn with its gold resources than the single one in Zagreb.

[Malcolm Big Head]

https://forums.totalwar.org/vb/showthread.php?t=77158

Not a text file just a post I read a while back. Getting old....memory fading...

[Snoil The Mighty]

Not sure if this applies to any circumstances in the OP or not, but who owns the territory the good is in also matters. If you have a trade agreement with the faction owning the territory you earn more from the same good.