Richest provinces in the game

[Zenicetus]

I'm only getting 468 from a maxed out merchant on Timbuk gold in my HRE campaign :/ The Moors own it and I have trade rights.

Speculation: Could the quality of the trade path from the resource to your capital make a difference?

I think the trade path and trade rights are background income, separate from the merchant's own income which is more dependent on distance to the capital, monopoly bonus, intrinsic resource value etc. Also, road quality (AFAIK) only impacts trade income for roads within your own provinces. Poor quality Moorish roads would affect the level of their background trade income, not yours. At least I think that's how it works. Maybe someone who has poked into the files can verify that.