Economy of Rome: Total War II

I think the most confusing part of Rome: Total War interface is city income. Real city income is revenue minus garrison cost. Income displayed by the game is revenue minus total cost of your army divided proportionally to city population. This leads to bull**** numbers, as revenue is not growing proportionally to population. So totally ignore city income figures.

You want as much revenue and as little spending as possible. Two most important revenue sources are sea trade, and land trade. Build best ports you can, and paved roads (or even highways) as early as possible, and get trade right with everyone. Trade-enhancing buildings like traders, market etc. are also very useful. Trade revenues grow only slowly with city size, so you basically want to have as many cities as you can, even if they're quite small, as long you they can build paved roads or port (that's level 2).

The second biggest source of income is farming and mining. Because it doesn't depend on city size at all, it's better to build basic farming and mining everywhere you can before you build advanced farming/mining. For 4000 you can build two mines in two cities, producing 400/turn. If you built mines+mines upgrade in one city instead it would cost you 5500 and produce only 350/turn. It's quite similar with farms - it's cheaper to have a little everywhere than to concentrate them all in one place.

Your next major source of income are wonders. Hanging Gardens give you +20% to farming income, unfortunately it's on the edge of the map. Colossus of Rhodes is even better as gives you +40% to naval trade income and is often barely defended by very weak Greek Cities faction.

Taxes aren't worth that much. There's some tricky equation to make them grow much slower than population, so big cities produce maybe ten times less in taxes than you'd expect. And in any case tax income is going to be much lower than trade income, and as higher tax rates reduce growth and public order I normally set all tax rates to Low by default. In smaller settlements it's important for the sake of growth, in larger settlements for the sake of public order. If you don't want to grow the settlement any more and it has high public order you could set tax rates higher, but it's usually not worth the bother.