The Great Harbor itself adds:
+10% tax (level 4 great harbor)
+10% tax (Harbor Office)
+5% tax (lighthouse)
+5% tax (road tolls)
+some miscellaneous trade value (your trade zone is probably already in the thousands of value, some small amount more is almost irrelevant)
So lets say +33% tax. This would be applied to a base tax of 46.3 per city for a fully upgraded city. For 5 cities that's looking at an income improvement of 76, or 6.3 ducats a month. With some good stewardship stats you might get 9.5/month.
Meanwhile, cost of a fully upgraded harbor is, what, around 10k-15k ducats? So it will take around 90-135 years to pay off. Though, you also have to take into account construction time. It takes almost a century to complete it, but the individual parts still pay off in roughly 90-135 years, or 110-150 years after you first invest in construction.
IMO it's a bad investment. Better is to just stick with the Great Wall. Even though you are a merchant republic, your cities still benefit from the Great Wall just the same. Indeed you can hold 6 out of 7 of the available holdings and get even more benefit than feudals (since they can't own the cities but you can own the castle).
For the most part I agree w/ you, & I do think G.Wall is very strong (in fact it's so strong that renders G.Fortress out of the picture, & I could easily do a whole post on that as well if you like) however I want to point a few things out, and with respect, I believe your numbers are off - firstly though, again let me re-iterate my strategy is to add GH to an existing tax strat, not rely on it solely - I'm not worried that it only adds +30% itself, because I have far more modifiers than just that.
Before the numbers though - I agree that trade zone bonus is pretty irrelevant yes, the moment you hit +40% city tax from it you require exponential increases to get further, in my screenshots I'm sitting on +46% but tbh - meh.
Here's a thing: Remember you add tax modifiers twice - once for all the county modifiers and then once again for all the character modifiers, and the important bit is that they stack on top of one another, this means that you can cash in on modifiers and then multiply that again.
Now, numbers. Let's take your 46.3 and I will do the maths twice - once ONLY using the +30.2% modifier from the GH (above), and once with that modifier, plus the 25.6% from my current town infrastructure, and the +46% from TV - and both times with my current character mods (which are stewardship 20 (I think I'm gaining over +85% in the screenshot but I can't check right now so I'll be fair and only add +75%), greedy (+10%) & I have a weeping statue artifact (+5%)). I am also NOT going to include my republic special holding (which is currently also getting the GH buff (so I am hampering myself here)) & I can easily get more modifiers (trade route/councilor tax bonus/other artifacts/more) than just this remember - again, it's part of a full bodied economy, not just a one off.
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1st time: Base (annual) tax: 46.3 now +(46.3*0.302)= Tax after 1st modifier: 60.2826 now +(60.2826*(0.75+0.10+.0.05))= Tax after 2nd modifier: 114.536
5* (for 5 cities) tax: 231.5----------------------------------------------------------------------------------------------------------------------------now *5 (for 5 cities): 572.684
/12 (monthly) tax: 19.291---------------------------------------------------------------------------------------------------------------now /12 for monthly income: 47.723
This is actually an increase of
28.432 per month, with just the one modifier but as this is only a part of the true economy strat here's the second number crunch:
2nd time: Base tax: 46.3 now +(46.3*(0.302+0.256+0.46)=) = Tax after 1st modifier: 93.4334 now +(93.4334*(0.75+0.10+0.05))= Tax after 2nd modifier: 177.523
5* (for 5 cities) tax: 231.5----------------------------------------------------------------------------------------------------------------------------------now *5 (for 5 cities): 887.617
/12 (monthly) tax: 19.291 (all these numbers stay the same of course)------------------------------------------------------------------now /12 for monthly income: 73.968
This is an increase of
54.677 per month
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Now, Cost of a fully upgraded GH - 1,900 for stage 1, stages 2,3&4 are each 1000 (before cost modifiers), each feature is 500-600, 8 of them so 4,000-5,200 = 8,900 up to 10,100 (not including cost modifiers, not including monthly cost (and tbh my opinion is that we can write the monthly cost off because it's essentially peanuts and you just wouldn't be building a great work if you couldn't already pay that cost)). Again, I'm going to only use the monthly profits from earlier with this.
Higher limit: 10,100/28.432 = 355.233 months before the cost is paid (29 years, 8 months)
Lower limit: 8,900/28.432 = 313.027 months before the cost is paid (26 years, 2 months)
Higher limit: 10,100/54.677 = 184.721 months before the cost is paid (15 years, 5 months)
Lower limit: 8,900/54.677 = 162.774 months before the cost is paid (13 years, 7 months)
With all respect Less2, this is a far cry from your 100yrs+ to pay off the cost. And with higher modifiers/more modifiers the cost will of course be cleared earlier. So with all respect to the Great walls, the great Harbor is NOT a bad investment, if you don't necessarily want to invest in it I can understand that - this is a very niche strategy, and only applicable for MR's, not feudals. IMO though, with these numbers behind it - the GH is worth it's weight in gold (but, granted it is hard to pick up on it without the complete and utter exploitation of it).
.png "CK2 income (breakdown).png")
.png "CK2 income (full).png")
