Yeah, like mentioned, sans talent it takes about 60, 65 turns for a trade pact (25 turns with 50 income pact)+dedicated emmissary (40 for a 2k follower, and you can probably nab cheaper) to pay for itself, after which is pure profit. Bit more if you factor in follower upkeep, but that usually isn't huge, bit less if you got a better pact income. It's not a major income source compared to a lot of the resource sites, but it's usually a lot easier to acquire and defend, heh.
Seriously consider pestilence mastery, though. Blight makes mines start producing necrotite (and hunting camps ichor, for what that's worth), which can bring in very solid prices on top of whatever the mine itself produces. There's probably more things like that I just haven't noticed yet, really...
E: Another cash money thing you can do I just noticed even existed, is
train workers. Buried in the economy tab, under show workers, you can click on a worker and then assign trainer. How this exactly
works is a bit opaque and there doesn't seem to be a way to tell what you're getting into when you hire a follower*, but as near as I can tell you need an otherwise unassigned follower, and when you click on it, it'll show you a list of what skills they can train. This only seems to work for a +1 on the available skills, but... +1 labor is a 25% boost in quarry output. +1 herbalism is free herbs coming in for that extra bit of dosh, on every site you have a relevant improvement on. +1 seafaring boosts anything coastal by 5%... and it's all
plus 1, not
to 1. So if you hire a labor 2, you can bump it to labor 3 just that easily. It's kinda' fiddly micromanagement,** but it can lead to a pretty substantial economic boost.
Plus you can apparently train workers that would normally not be
able to gain extra skill levels, i.e. demonic wretches, and I assume fae bound or skeleton laborers.*** That's extra huge since they might as well be free and you can easily get a good half-dozen of them without much trouble... their biggest downside is set skills and no ability to improve, and, well... this gets around both of those, to an extent. For such an impactful task, it's kinda' surprising it's as buried as it is, tbh...
*There really should be, dev, if you're reading this

** Follower task to just go out and train every available worker would be ace, just sayin'.
E: *** Steam forum suggest those two actually can't be trained, just the wretches. Interesting the demons would get that kind of advantage, really...