It depends.

In my current game as Turks, moving the capital to Caesarea actually maximises the total income by aout 750-850 depending on harvest quality. But I don't yet have merchants in Timbuktu. I'm focusing on monopolising the amber market, first. It's closer and it's only 1100.

Next will be the iron and gold markets.

I'm not sure if moving the capital to maximise only trade income would be financially wise. For this empire I am runnning, it would'nt be. Perhaps with high level merchants monopolising gold it would but I'm nt sure.

One thing I am sure about is that moving the capital to maximise overall income will always be financially wise.