To specify Salt Caravan's effect: For the cost of 150f 150w 150c you receive a trickle of coin at a rate of .1 coin per sec for every 100 food or wood in the bank. So, if you had an average of 600 food banked for 1 minute you'd earn 32 coin, the same as an un-upgraded villager. Food and Wood are counted in the same total and weighted equally, e.g. 500f + 500w = 1 c/s. Furthermore, values between 100's are included in the rate calculation, e.g. 653f = .653 c/s.
Soldier finishes researching Salt Caravan at 16:31 in-game and the game ends at 27:05, giving just over 10 minutes of data. I employed a crude method to estimate the total coin earned. Every 5 seconds, starting from 16:31, I recorded the food and wood banked and calculated the trickle rate. Then I determined the coin generated for the next 5 seconds until the next recorded data point based on that rate. Attached are my data. This graph shows the results:
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Setting aside that each resource is worth a different amount of villager seconds and treating all resources as equally valuable, there are two ways to analyze the cost of Salt Caravan. One is to treat the upfront cost as a total investment and say that the tech will have paid off once it makes more than 450 coin. The other views it as costing 150 coin to convert 150 food and 150 wood into coin over time before generating coin; in other words, it's treated as though the food and wood spent on Salt Caravan were instead sold at the market, only the rate is improved at the cost of time. In the latter analysis, the initial investment of 150c was paid by 2 minutes and from then until 8 minutes the 150 food and wood was turned into coin.
I also don't mind leaving out the cost of the TP from this analysis, as I think it's reasonable to expect players to ally with Berbers to train Camels, boom with Nomads, or even pick up the Salt Camel when natural mines start to run out. Therefore, whether to research Salt Caravan is almost always a secondary consideration.
Some limitations to mention:
1. As this was an inexact way to measure the coin generated, this is bound to be off from the true total. If a large quantity of food or wood is spent just after the total is recorded, the trickle rate will be greatly overestimated. Likewise, if the total food and wood suddenly and dramatically increases, then the rate will be underestimated. Hopefully these effects were mitigated by the frequency of the increments that data was recorded, as well as the occurrence of such errors being somewhat equal in either direction.
2. While this was a real game played on the ladder, Soldier was aware of the effect of Salt Caravan and adjusted his macro to take advantage of it. Consequently, the total may be somewhat inflated, and this example should be construed as showing Salt Caravan in a positive light. And while Soldier was put under some pressure, much of the later part of that game was rather passive. Any conclusions drawn are limited as this was the only game from which the data here was derived.
It's also worth pointing out that especially towards the end of the game Soldier was probably banking too much wood and there were likely more optimal ways to actively convert wood into coin than chopping more wood. Outside of just selling it, a better option may have been to make 1 or 2 Estates, especially since as Haudenosaunee/Iroquois he had access to 1k coin via the big button. Or he could have taken more villagers off wood to gather from the salt mine he just constructed. As an aside, I've noticed player's first instinct when trying to maximize Salt Caravan is to bank wood, even though food also counts towards the trickle rate and is gathered faster in most cases. Why is that?
Overall, I think people's initial impression of Salt Caravan was probably mostly accurate but maybe a bit too low. For 450 resources, Soldier received on average 2 un-raidable pop-less villagers, though that fluctuated between roughly 3 villagers and half a villager. While inconsistent, that compares somewhat favorably to Capitalism, but lags behind Stagecoach, Pennsylvania Pound, and the Dutch Bank. Doubtless Salt Caravan would've looked more impressive if the game lasted longer, but if the game ended earlier, say if around 21 minutes into the game Soldier failed to hold the opponent's push, it wouldn't even have had the time to pay itself back. On the other hand, Soldier could have researched it much earlier; even from just his macro in this game Salt Caravan was affordable as soon as 6:30. Taking the upgrade earlier gives it more time to pay off, but it can be tough to manage. Even though it isn't that expensive, roughly the cost of 3 Forest Prowlers and a longhouse, every incomplete batch and every second of TC idle time makes all the difference in the early game. To me, this reads as a situational tech, but not unusable.
I think players who are already going for a greedy opening and training Nomads shouldn't be afraid to pick up Salt Caravan early. It's also worth considering when full booming in the late game. But in either case, it's not really a tech you need to lean into all that much. Rather than central to any strategy, Salt Caravan should be viewed as a helpful boost in the background. I think you'd be surprised how much food and wood you bank naturally over the course of a match, whether it's while prepping unit batches, saving for an expensive upgrade or age up, or just on accident in the heat of the game.
*https://www.twitch.tv/videos/1165856926
Edit: Helpfully, Age of Streaming has uploaded this match to YouTube so it is still viewable
https://www.youtube.com/watch?v=3_nUkCgzdVY