levees, yes. if you're playing as the dutch, their dike UB replaces the levee, and it can be built on river cities and on coastal cities.
GMs, with him selected hold down shift and right-click on another civ's city. then hover over the trade mission button and that'll tell you how much you'd get if he was in the city this turn and could do it. it may change a little bit by the time you get there, if the city goes up or down in pop or builds a harbor or whatever but not drastically unless he whips half the pop away.
if you're playing warlords or BtS, often the city that has ToA gives far more than any other city, so i always check it even if it's small. generally, the further from your capital the better, and the more commerce the city has the better. which means more population and more trade-helping buildings in the city (harbor/custom house stuff) the better, so coastal capitals are often real good.
If you hold down Shift then right click on the target city then mouse over the trade mission button it will show how much the trade mission will be worth just remember not to let go of shift until you cancel the order by right clicking on water. You can do this even for overseas cities. And the Great Merchant must be in one of your own cities for this to work. The best trade routes come from Coastal/Large/Temple of Artemis/Overseas cities and obviously from a combination of all of the above.
Sorry I dont know the formula.
And yes Levees need to be next to a river, or on a corner.
ToA = temple of artemis wonder. not applicable to vanilla.
the palace gives +8 commerce, and your (and the AI's) starting location generally has a lot of food, so the capital generally has good trade routes so often more gold. rex = rapid expansion, which can be painful on the pocketbook!
oh good point, i didn't say that because didn't even know it, thanks! just tested it, actually he has to be in your borders, doesn't have to be a city. you know, in case he's in an anti-social mood or something.
As it happened one game, I had a GM reach the Sumerian capital the turn before he completed the ToA. So I waited one turn, and the result changed from 2250 to 2850 . That at least gives you an idea of the boost - and helps explain why many (most?) civ players allow the AI to build the ToA.