Settlement details screen will have "ghost" trade icons that reflect expected future income from building. Choose to build, then check details. There will be a new trade icon, and when you mouse over it, the game will show you the new trade income from the market.

Actual trade income will vary a bit (e.g. blockades, sieges, pop growth, etc), but the details screen will give you a reasonable estimate of the value of the market for the next 20 or so rounds, assuming no drastic changes (e.g. if enemy takes over next door province, obviously dramatic change in income).

You will see that adding trade buildings adds tiny %s to trade income. Typically return on investment will not occur in 20-30 rounds. (And this is not taking into account the rather steep inflation in the game; "real" returns are thus sharply lower) The very best trade provinces (e.g. carthage, rome) will sometimes give 10% (10 turn) returns on the FIRST level building. At higher levels it might take 50-100 turns to earn back the cost of the building in lower trade provinces.

Ports in contrast usually pay their own way within a few turns. The best provinces wil earn 1k+ from a single route, meaning the port will pay for itself in < 1 turn!