There should be a Menu Item within the Market to view the supply and demand of goods across multiple ports. The menu item would open and show a list of all available goods (Cotton, Iron, Rum, Tools, etc.). Clicking the good will open a supply/demand chart for the good showing the ports the good is sold from and purchase price, and all ports that demand that good and its selling price. The supply/demand chart should be limited, showing costs for only ports that you have warehouses in. This would make it easier to build and update trade routes.
I have a few alternative suggestions and add-ons for this that I may post later.