Since some people ask about the value of buildings sometimes, I'm gonna share my 2-cents from a mathematical perspective.
Some people say, hey it's just +0.1 or something for 100 gold! So it takes 1000 months for a full refund!
Well yes. true. BUT when you have spare money lying around early game, it's better to spend them.
Don't forget, even if you have 1000$, and you're losing 1 decat a month, you will keep losing money. Spending them on buildings will stop the bleed long term.

Full Version HERE!

In Case You Come for Manufactories!

In short: Tax is for early game.
Tax should give most income when you're not dominating trade & trade goods are of low value.
Later on you might get tax income modifier and with a rather high base tax empire, tax income is pretty sweet.
As a rule of thumb I find it's best to spend that 100$ on provinces that give +0.10 or above improvement. Any lower is not worth it.

Needless to explain.
They buff your trade power in a province.
Nothing magical about it, it gives you more trade power % in that node the province belongs to.

More trade power % means bigger piece of the pie from the pile of cash in the node.
Best be built on trade centers ie: Constantinople, Danzig, Wien, London etc. you can see it on trade map view. One trade center is worth at least 10 province of trade power.

Works similarly as tax buildings early game.
they buff the production income of province depending on base production & trade good value.

Similarly, I find it's best to spend that 100$ on provinces that give +0.10 or above improvement. Any lower is not worth it.

BUT, this is part of the magic that you will learn about below.

It gives flat +1 Production to the province and generate income boost from base trade good price.
It is the most expensive building you will be spamming.

It usually gives the misconception that it looks not really paying off because of its high price. Wrong.

Except, starting from mid game it becomes increasingly important.
It costs 500$, which means I usually recommend building them if only they give +0.5 ducat buff per month.

Here is the catch, unlike tax, the production value of some goods actually increases/decreases with time!
For example, Iron, after the event of “Iron Working”, the production value jump from $3.0 to $4.5, which is a 50% increase. And with a manufactory, you can easily get over +0.7 ducats boost per month.

Entire list of Price changes throughout the game found here:
http://www.eu4wiki.com/Trade_goods#Permanent_changes

Here is another catch, manufactory first gives a flat +1 to production, then production building stack with it, by giving it another +50% (late game +100%).

Which means, using our previous example of Iron.
If you building a manufactory first, the game will tell you you get +0.6 boost, then you building the production building, the game will tell you that you will get +0.2-0.3 from that 100 ducats.

In total, you will be getting nearly +1 ducats boost from 600 ducats you’re spending, which is one of the most efficient investment you can get!

The value increases with time as events fire,
And every few admin tech gives a percentage increase to production efficiency. So your income actually scales into late game.

If you start in Africa or Americas or even China/India, you will have extremely valuable trade good like Dye ($5.0), Coffee ($4.5), Tobacco ($4.5) etc; and last time I played Kongo, I first built a production house on Dye, then the manufactory gives me another +$1 increases per month!

In short, Production scales into late game by itself.
1. Admin tech - production efficiency increases
2. Prices increases overtime by fixed events.
3. Two buildings stack on each other.

Here is another benefit of relying on production, it boosts your trade income indirectly.

On top of your production income you already get right off the bat, all the production money that are produced are summed up towards the money in “Local Trade Node”.
If you look at the trade nodes, you have money coming in, going out & local.
Local is the local production from all provinces in that node.

Let’s say you control 2 nodes, for example, Ragusa & Venice.
Assuming you have a considerable trade power in both nodes, so normally you would move your trade capital to Venice since it’s downstream, and use your merchant to push money downstream from Ragusa.

So here is the thing, the money produced from local Ragusa is going to be collected once again in Venice!
Since trade is about pushing money around, if you push the money from Ragusa downward to Venice, it is considered trade money, regardless who’s produced it.

Of course, in this sense, your produced trade money will also split by some other powers in the node, but after all, you’re collecting TWICE from your investment basically!

So you can probably see already, production buildings boost more than simply production money, which makes them so valuable.

As your entire economy rely more on tax and production, it will also lower the inflation impact of Gold mines in your country, because the inflation you get from Gold depends on the percentage of Gold on your total income.

Anyone who played Aztec/Serbia/African/Indian country knows, inflation CAN ♥♥♥♥ up your entire economy by making literally everything super expensive.

6.1: Forts:
Needless to explain.
Give you defense advantage in wars.
Except it’s worth mentioning that, up-to-date forts give you army tradition.

6.2: Manpower buildings:
More manpower never hurts.
I prefer to spend on provinces with over 500 manpower boost per 100$ as a general rule.
But of course, in places like Africa you have to lower your standards.

6.3: Force Limit buildings:
Useless. Waste of building slot.
Generally the limitation to your army size is supporting it without losing money, not the hard FL.

6.4: Docks:
Useless. Sure cheaper and faster ship building is good, but it’s not like you can’t wait for it to be built.
And if you’re using heavy ships instead of galleys you will have an impressive enough fleet before you are nearly half of your navy force limit.

Bonus: Heavy ships barely die compared to galleys in combat. So you don't have to keep rebuilding them all the time.

Think of it this way, money makes money,
If you don’t build the buildings there is so much you can earn; but if you do, you will get more money to build even more buildings, and more income.

So it just snowballs your empire into an unstoppable money printer.

Thanks for reading.

I hope I might be of help to clearing some things up, given I had been in the same situation where I just think buildings are overpriced and doesn't even refund its cost in decades.
After playing more and more games, I realize buildings are strategic, and can make a huge difference from counting 0.01 ducat income, to an empire of money printer!

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Dismantling HRE by 1600 Using League War - Early Germany
Vassals: Tips for World Conquests/Ultra Blob Expansions
The Three Mountains for Everyone (Guide/AAR)
Guide to do Easy WC/1-Faith, Revoke Privilegia in 1530
Guide to Stacking Discipline
Guide to Buildings & Income
100% Mercantilism & Venetian Sea
Guide to Keep 75% Autonomy for Territories After Raising it for Lower Unrest

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