I wonder how much do you rate shields and beakers compared to each other. It occurs very often to me to wonder if it is favorable to build a courthouse or a factory in some city and I cannot estimate properly when the investment is going to pay off.
Courthouses do not make a lot of sense in core cities, but factories do, if the city is producing a lot of shields. So, I do not think that this is actually a trade off which you have to do often. Since factories increase your shield production by 1.5, factories usually pay off after 2x the turns it takes to be built.
But shields and beakers, the question of balance keeps arising too often and is not that wide. Assuming reasonable circumstances (somewhere on the end of 2nd era, beginning of 3rd, no special need or lack of need for shileds in the cities, reasonable science balance in the game etc), how much do you value one to another? Obviously, we are building or going to build mil units in the city, not wealth. In wealth, calculating is simple. From my experience, I would say that I have reached the conclusion that a single shield is worth somewhere between 2.5 and 3 commerce, but I would not be surprised if my instict has lead me to a completely wrong conclusion....
In case of militaryunits, I would say that it far exceed 2.5 or 3 commerce. And the reason is simple: military units heal, and stacks are overpowered.
Imagine you have an army to invade the enemy which is just strong enough to do the job. After the war, you will have almost no army left. If you now increase the army at the beginning, you will have much more units left at the end of the war and they will be able to heal.
Take a knight with 1/4 HP as an example. As a fighter, that is basically useless, but through healing it will have 4/4 in three rounds. Basically, a knight which costs 70 shields and which has the ability to heal 1/4 of its HP per turn, will have some 'implicit' shield production of about 23 per turn. In the meantime, through fighting, these units will give you more land, resources and gold, as well as technologies, if you demand them for peace.
On such higher difficulties, you would be using most of your commerce on teching. However, you can't turn shields into beakers, so what other ways do we have of calculating such ratio?
You can by building libraries & universities. You know how much they cost in shields and their maintenance, and you know the cost of all the techs.
Since libraries & universities give 50% higher science output, you can then calculate the beakers you get per shield.
Next to this, we can use our gold to upgrade units, which is a 3:1 ratio without Leonardo's Workshop. Given that on higher difficulties, you usually aren't able to obtain this GW on larger maps until far later in the game, I think using a 3:1 ratio is fair. Yes, this still doesn't turn beakers into shields. But we can however allocate gold from our treasure to such upgrades, rather than them being turned into beakers. So this would give a beaker to shield value ratio of 1:3.
We can also use gold to hurry production if we need money for it. And on higher difficulties, we should be using some of our treasury on it. We do need to be in the right government however. Especially when playing as agricultural civs, I use quite some of my gold to pay for aquaducts. I also use it for settlers now and then. Just like with upgrading units; yes, this doesnt turn beakers into shields. But it does allocate gold from your treasury towards shields, which is pretty much the same. This gives a beaker to shield value ratio of 1:4.
If you are cash rushing on turn 1, you pay 8x the shield cost of the unit or improvement, so then you would get another value and there might be a reason why that change was made. However, even then the exchange is not really 1:4 or 1:3, since you often have excess shields.
Consider for example a city producing a horseman, which costs 30 shields, with 9 spt. If you rush the unit when you have 18 shields, you pay 12*4=48 gold. However, you would actually have used 36 shields to produce the horseman, with 6 shields wasted. So you actually save 18 shields, and you are paying ~ 2.67 gold per shield instead of 4.
Exactly. Let's take a closer look at the space race victory: The space ship parts can be built easily the turn you finish the required tech. You just need to start the pre-build early enough. So production (shields) are not the problem. The bottleneck for a space victory is the speed with which you can get all the techs. --> Beakers are much more important than shields.
But there is a "cap off point": at one point you will reach 4-turn research, and then any additional beakers don't do anything anymore! They are just completely worthless. So even during the same game, at different points, beakers can be most important or completely worthless...
No one doubts that faster technology might at times be better than having more shields. OP asked for some kind of exchange rate to help in the decision making. Refering to cases in which you simply know that you do not need one of the two does not really help, because in that case you do now have to make any decision, since you do not have to give anything up. In your example, you might have no problems with getting the shields you need, but you will still need shields, and beakers for tech. How many shields are the space parts? 3000-4000? And how many beakers do you need for the technologies? It depends on the difficulty, but they are certainly several thousand.
