1. I build all port upgrades ASAP. They pay for themselves easily.

2. Don't underestimate mining income. They show up as goods in the trading screen as well though I don't know how much they add to trade.

3. Markets not only increase trade income in the settlement you built it on, it also raises trade income in all the other settlements trading with that settlement. It has a good rate of return when you look at it that way.

4. The increase of each farm upgrade is 80. That's not too bad. The first few upgrades have a good rate of return. Their main purpose, especially the later ones, is to increase population growth and the maximum population allowed. Many cities won't reach 24,000 or even 12,000 without farms. That's the same rationale for the 1st and 4th market upgrades, though the 1st is cheap, anyway.

I built all the port, market and trade caravan upgrades I could as Parthia. Antioch, Tarsus, Salamis and Jerusalem are all earning 5,000+ and Sidon is earning 4,000+. Most of that is trade income.