Exact bonuses to Trade from Buildings?

Playing as Venice, I am trying to create a coastal trading empire, with my trading ships filling the Mediterranean. However, as I build up my Ports and Markets etc, I notice that they do not seem to give much of a bonus to my income. In some places earning over 1,000F in trade income, higher level markets costing nearly 10,000F seem to boost trade barely by 100F. It seems that the Port (not military version) line of buildings is the best way to boost trade income, due to the extra trade route provided by each level.

Does anyone know the exact bonuses given to trade by the market, road, port etc line of buildings?

I happen to have done some research on this... but the numbers are so widely varied that it's nearly impossible to be able to tell. It depends on... pretty much everything, quite complicated. Sorry buddy.

Do you know if Markets only boost land trade, and Ports only boost sea trade? That would explain why markets don't seem effective in my coastal Venetian Empire.

That is true from what I've found. Markets are designed to affect land trade and ports are designed to affect over-sea's trade. Although I like to keep the two parallel.

later youll need high level market buildings to build artillery upgrading buildings.

On land: paved highways give the main boost to trade (not on islands though). In the sea: the harbors. But the latter is true only if you have other harbors to trade with (harbors in provinces that lack the resources you can trade and that belong to factions that are not at war with you and optimally, that belong to factions that you have trade rights with).

If you lack the harbors to trade with (or the harbors you have trade with has less trade routes than yours), some of your trade routes will be "idle". Check in western sahara: you can build a harbor there, but there will be no sea trade in the province (at least until the ocean-going age. and i have not played to that point yet...). Markets, Print-Presses, Banks - all give considerable boosts to trade, but only if the province has significant resources to trade to start with.

Also, in v 1.02, if a general sits in a city without a basic market building, he is highly likely to earn "ignorant" and "incompetent trader" traits: these traits appear even if the first building built is a road or harbor (rather than the corn trade shack). It seems, the game wants the basic trade building to be the first to be built. Both "ignorant" and "incompetent trader" traits can hurt trade considerably. Especially in the resource rich provinces.