This occurred to me last night as I saw the happiness in a settlement drop below 50 again (yes, because of a bug but still).
When I find a "caps stash" (without perks) it's usually less than 30 caps. The most money I can recall taking off raider boss is 110 caps. A clean suit of clothes costs 11 caps, and a good solid handgun around 25. When raiders kidnap someone they usually ask for 300-400 caps as ransom, a sum the family can't easily come up with.
From this we can infer that the average wastelander takes in something like 30-40 caps per week and 350 caps represents a family's life savings.
With that in mind I'll observe that purified water sells for around 15 caps for a unit weighing half a pound, which if you do the math works out to roughly half a pint. Which means that a gallon of fresh water per day (the usual recommendation for good health) would cost 240 caps per day, or six weeks pay. By comparison, the median income in the USA is $51k, so it would be like a gallon of water... one day's supply... cost us around six thousand dollars (I'm using round numbers here, obviously).
Where I'm going with this is that even a little fresh water is a big ticket purchase to almost everyone living in the Commonwealth. A steady supply of it is a luxury inconceivable to most all of them, something reserved for the Zuckerbergs, Bloombergs and Soroses of the wasteland.
And our settlers, the lucky few dozen who take up with us, have it on demand. As much as they can drink, right there at the purifier. We water our crops with it for goodness sake, which means they also have access to better food. That's before we get into the security provided and the fact that many of them are given high-paying, low risk jobs running our shops.
If happiness in any settlement drops below 90% we should be able to poll our residents and boot out the ones that aren't happy, because there must be thousands of poor souls out there who'd kill to be in their shoes.