Intially, with just one province and a few ships, trade may not seem worth it, and will probably not even pay for the upkeep of the ships. But, as you increase the length your trade routes the amount of money you make from each trading post will go up exponentially.

What you want to aim for is to have all of your coastal provinces equipped with ports and trading posts, and to eventually have a ship in EVERY SEA LOCATION ON THE MAP. The amount of money you will make is staggering.