Trade Route Money Calculation

[binhthuy71]

Yeah but they cost 3gpt, what sort of requirements do I need to build one?

You need to research Compass in order to build a Harbor in a coastal city. The maintenance cost is one GpT higher than the other buildings of its era. The trade off is that the Harbor gives you +1 for each sea resource worked by the city. For what it's worth, I always build a Lighthouse in a coastal city (+1 on every ocean tile and +1 from each fishing boat) and I hold off on the Harbor until I have some fishing boats out.

 

[burleigh]

Though the harbour costs 3GpT, that's the equivalent of 3 road hex tiles maintenance. If your typical distance between cities is more than this, then probably the harbour cost is worth it (especially if you have some lovely fish or whale blubber to fry in there as well)
I'm not 100% sure on the next point, but I also think the harbour counts as a rail connection once you get railroads for the production bonus

 

[Mesix]

Yeah but they cost 3gpt, what sort of requirements do I need to build one?

Build a harbor in a city which has fishing resources. Then you are paying maintenance for the added . If you also build a sea port in the same city then you will get additional for each resource to offset the maintenance cost and +2 total from each resource. As a general rule of thumb, I like to have at least one city on each continent with 3 or more fishing resources to build my lighthouse, harbor, sea port combination.