Another thing I haven't seen mentioned: With Solver's patch, Mining Inc can very easily make you quite a lot of money. A simple example: you have two production cities with 20 base hammers each, and forge+factory+power (for a total output of 80 hammers over the two cities). You have Mining Inc founded in the Wall Street city, and it has enough resources to bring in 20 hammers per city. Expanding it to those two cities will probably cost ~30gpt (after the bonus from the HQ is accounted for), and double the hammer output of both for an extra 80 hammers per turn.
If one of those cities now builds Wealth, you will be looking at a net increase in gold of 50gpt (80 hammers-30 maintenance), but you will still be generating the same amount of hammers (and even better, the hammers are now focused in a single city). You can also alternate which city runs wealth while the other builds any necessary improvements or whatever, and when you really need to ramp up production, you have an extra 80 potential hammers that can be assigned at will.
So Mining Inc is well worth spreading pretty much everywhere that you can afford the health for production buildings.