In order to determine if a trade route is profitable you need to ask the trade master of the post you wish to sell at. To do this simply type "value <'trade good'>" you can also ask the trade master how much certain goods will sell for when brought on a specific trade carrier.

For example
Value emeralds:
We will pay you 23130 gold for each casket.

Value emeralds mule/covered/train:
A mule can carry 11 caskets of emeralds. We'll pay you 254,430 gold for that.
A covered wagon horse can carry 85 caskets of emeralds. We'll pay you 1,966,050 gold for that.
A wagon train carrier can carry 125 caskets of emeralds. We'll pay you 2,891,250 gold for that.

It is also possible to mix your freight values to fill up the empty spaces.
value tar covered salt will return this result:
A horse pulling a covered wagon can carry 8 barrels of tar packed with 9 barrels of salt. We'll pay you 1,680 gold for that.

When freight is sold the value will more than likely decrease as you fulfill some of the post's demand for the item. The next person to sell this same freight will receive less gold.

Valuing item by item can be quite a chore so some very skilled code writers have written scripts for valuing all trade goods. Ask around to find where to find these scripts.