A set payment to build infrastructure that would be regularly charged to gradually increase port strength. For the turns, gold, and influence spent could grant bonuses such as:increase/adds a population growth modifier (Spastic's idea), resource productivity ( 2.5% per level) and trade bonuses (gold).

This payment will be daily/weekly?

I like the general idea but I need some more details. How exactly you will decide what to spend and how can we make a standard amount of gold to affect something else proportionally?

This sounds more like a feature and not a law.

I think it could divided into multiple laws. I shall start with one of them.

Municipal Improvements- This would symbolize a nation's port having been upgraded from little infrastructure to more developed towns and cities. It would include improvements in sanitation, medical, or municipal services. The suggested cost would be a 1/2 gold piece per port citizen from the national treasury every three months. So in a port of 30 million, every three months it costs 15 million or 5 million monthly. The benefit is a 1% increase weekly in population and halves the power of all Black Deaths used upon the improved port's citizenry.

Way too expensive to pay half of pop. A port woth 30mil pop makes 300mil gold per year, if all citizen are paying, which they probably wont be. The port therefore makes 25mill per month. Deduct 15 mill of that and the nation is left with 10mil. In my very limited experience not enough citizen will be paying to get 15mil or much more in a port of 30mil people.

Instead there should be options to pay for upkeep/expansion of infrastructure but players/nations should be able to just pay month and build more stuff in the port, much like building up the port in the court menu. Could also work like the warehouse paying system

The amount, for lack of a better term, of infrastructure effects how many paying citizens there are or alternatively port population growth.
Upkeep/Expansion keep the infrastructure from deteriorating as well as keep population alive. Also steadily expands infrastructure depending on how much gold paid.
If a nation doesn't pay upkeep a popularization death modifier should grow. Pay for upkeep decreases the mod.
Both paid monthly.


Laws
Port upkeep
How much upkeep to pay - 1-100%
Port Expansion
How much money spend on infrastructure expansion - gold amount


This also provides a possible answer to the possible problem of having massive over unrealistic port populations.

Well its open to discussion. The numbers are up to you guys, I just offer ideas.