I'm not sure if a fixed income bonus fits the fixed income system in place. Right now, the sum of all the walls' upgrades' fixed income bonuses (1300) and the sum of all buildings requiring maintenance cancel each other out. This may be intentional, so giving the trade markets a fixed income bonus might throw things out of wack. Then again, Pergamon's Un/Supervised Hellenic Administration equivalents give fixed income bonuses...
A possible solution that follows some rules of good gameplay is to drastically buff the trade_base_income_bonus. According to
https://forums.totalwar.org/vb/archi...p/t-70877.html, each point increases the current trade by 10%. This kind of multiplier forces the player to make choices: should I spend a huge amount of mnai upgrading this region's trade by a multiplicative factor? Does this region even have good trade income? But if it does, would building higher levels of markets destabilize the region?
The math regarding trade vs corruption can be a bit weird. In general, a +1 trade_base_income_bonus increases income by up to 150 for a decent 24000+ size coastal settlement, while -1 law decreases overall income by 150 for a faraway settlement. So the max level of markets (+5 trade, -3 law) would normally give just 300 mnai. Of course, many other situations can arise from this.
The settlement could have less than zero law, so the law malus would have no effect on this lawless hellhole. You'd get the 750 mnai trade. Of course, if this settlement is far from your capital, the corruption would become obscene, but corruption can also be capped. On the other hand, if this settlement is right next to your capital, then all the more power to you.
I personally find the law penalty good for gameplay reasons. It forces the player to think ahead where to build clusters of markets in their empire. However, the trade_base_income_bonus should be buffed to increase the number of settlements in the average empire that would generate more income from a higher level market. The proposed implementation is: +1 trade for levels that don't introduce a law penalty, +2 trade if it does. Overall, level 5 markets would net you +8 trade and -3 law. For a huge city not too far away from the core of your empire, that's an easy 750 mnai per turn. The total cost of upgrading to level 5 markets is 36000 mnai, so this is roughly the equivalent of upgrading a mine to level 2 in the Iranian Plateau.
What do you think?