It is very important to have in mind that, when you deliver a commodity caravan/freight, you get 2 results: One time bonus + continuing trade route.
1) Continuing trade route
(trade that is added to the 'base trade' of your city; 'base trade' = trade arrows provided by your workers; 2 arrows/ocean square, for example)
Each city has 3 slots for commodities offered and 3 slots for commodities demanded. In the early game all slots are open and you can choose which of 3 commodities available in your source city will be transported by your caravan.
On delivery, 2 trade routes are created between your source city and the destination city (which can be either yours or foreign). One trade route represents your caravan (let us say silk that you have built in Rome and delivered to Paris) and the other trade route represents a caravan supposedly built in the destination city and sent to the source city (let us say wine built in Paris and delivered to Rome).
Later on, when slots are blocked, a new trade route will be created only if its value is higher than one of the trade routes already existing in the city (max = 3 trade routes/city; never more)
Very important: The value of a trade route DOES NOT DEPEND UPON THE COMMODITY DELIVERED OR THE DISTANCE between source and destination city. It depends upon the sum of 'Trade in Source City' + 'Trade in Destination City', with a number of modifiers (foreign city, different continent, road, RR, ...).
2) One time bonus
This is the amount of gold that is added to your treasury on delivery of the caravan.
Very important: A NUMBER OF BEAKERS IDENTICAL TO THAT AMOUNT OF GOLD IS ADDED TO YOUR RESEARCH BOX. In other words, you get 2 'One Time Bonuses': one in gold, one in beakers.
Also very important:
The value of the One Time Bonus DEPENDS UPON THE DISTANCE between Source and Destination City (and there is no upper limit, so the farther the better, BUT this can be a lengthy affair: 15 turns for delivery 45 squares away from your source city onboard a caravel, hence the value of LH and Magellan).
Also very important:
The value of the One Time Bonus also DEPENDS UPON WHETHER THE COMMODITY DELIVERED IS DEMANDED by the Destination City.
The most complete and accurate formulas are to be found in samson's thread in Apolyton's GL, but there is a problem with access to Apolyton today, so I can't give you the link now.