Which are we talking about here? Craft buildings? Or Inns? Mines? For the latter two, it's always worth it if you're willing to spend the time to wait for the payoff. You say mines are bad, and I say they are great. I had one province making 13000 a turn thanks to its gold mine. A 500 gold increase for 10,000 gold pays itself off in 20 turns, but this is not counting the increase in taxes from a flat percent bonus to your income. The Geisha 10% tax bonus will turn 13000 into 14300. You will see a bigger payoff in that regard depending on the amount the province makes by itself.
For the former (craft buildings), that will depend solely on your trade routes, which can swap hands and change at any given moment during a game. You will get something from Britain, France and the US, but I don't have solid numbers on those.
With 4 "years" (in reality, like 40 turns) from the end of the game, I was making 25000 gold a turn, and had so much money I couldn't spend it all. All I really did was get my towns up to level 2, build a station and a market, and then upgrade whatever trade resource they had, in addition to building a port.
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