The values for all the trade goods are set in the startpos file for each era. Their locations are also set there. They range in value from 20 to 50. I am sure that these are themselves percentages of 52, as the build_prod file has Cathedral income and Copper mines valued at 100 and they produce 52 Florins each.

Each province supplies its tradegoods to every other province with a port that doesn't have the same tradegood available from within. This is why Antioch earns such a huge trade amount in the game. It is the only coastal province with Spices and Gems, both worth 50.

You can create a Supply & Demand by limiting the availability of tradegoods or altering the value of the rarer goods, but there is no way to set up distinct trade routes.
I remember in games like Ceasar 3 where your trade partners had exactly this. Naples has demand for 48 Marble, Syracuse supplies Marble and the same for your city, your citizens could demand Wine and you'd have to buy it from others to keep them content and attract the upper levels of society. Civ2 also had a direct city to city supply and demand trade caravan/route model.

Maybe in Rome.