There are already a few port building suggestions about production oriented. Lets use this port buildings to effect trade routes and prizes.
For example:
i have a miner in gasp and producing iron. which will increase supply of gasp, with enough supply the prize will go down in gasp for sale and this will help to make gasp more popular route for traders.
or make a cigarette factory or whatever else:d in gasp. that way you will consume tobaccos in gasp's warehouse which will increase tobacco demand and prize in gasp. this will also help thorakas and psaral ports due to their tobacco producing.
This will open a lot of opportunity for trades. and need better understanding and judgement for both nations and guilds. for example tool sale to akrotiri will make it a lot more profitable. still it can have some caps but lets say akrotiri pays 30c for tools because they have a lot of workshops in their ports and need tools desperately.
I know this isnt what you suggested Haron but i hope this one will help your suggestion.