I don't understand this much better than you do, but: tiles do not produce
gold, they produce
commerce. From what I can gather, commerce has little effect on your gold balance, it impacts your research points (beakers) per turn instead. What I really don't know is why.
If you've played Sid Meier's Alpha Centauri, this system makes perfect sense, the only difference is the name of each thing, which is probably clearer in Alpha Centauri than Civ 4, unfortunately.
It's really important to understand this distinction, so I've replied to it for you.
Anyway, here goes.
Tiles never give "Gold" (in Alpha Centauiri, "Credits" iirc). They give "Commerce" (in Alpha Centauri, "Energy").
What happens to all of this
Commerce is up to you the player, and your choice is determined by
where you set the research, espionage and culture sliders, which make
beakers, espionage points, and culture, respectively.
Beakers are used to research things,
Espionage Points to get passive espionage bonuses and to pay for active spy missions, and
Culture to "pop" borders, win cultural victories, and keep citizens happy.
Note that the three sliders can add up to less than 100%, though they cannot exceed 100%. In fact, if you want, you can even set all of them to 0%. This is because any left over percentage is automatically used to make
Gold.
When you see that you can only set your
research, espionage and
culture sliders to a sum of 60% of your
commerce before your
Gold goes negative, this is because your empire requires 40% of your
Commerce to be converted to
Gold just to pay for its upkeep costs.
The previous paragraph is where most people get lost, so, an example:
Your empire has enough cottages/hamlets/etc to generate 100
Commerce per turn, with maintenance costs of 30
Gold per turn.
How much gold can you make? How much can you research? Let's find out:
First, some buildings like libraries and banks give bonuses to certain things, so we make another simplifying assumption:
Every city has a library, for +25% research, and every city has a market and grocer for +50% gold.
So, with that in mind, let's "play" with the slider.
Setting research to 100%: This means every bit of that 100 commerce is converted into
Beakers. This means 100
commerce becomes 100
BASE Beakers. Note that I said
BASE Beakers, because we still have to factor in the libraries, which boosts the 100
BASE Beakers into 125
Beakers total (we get a 25% bonus, 25% of 100 is 25, so we make 125 total).
So if we set the slider to 100% research, we generate 125
Beakers, but because we have not converted any of our
Commerce into
Gold (we diverted it all into research, afterall), we have aren't making any
Gold to pay for our maintenance costs, and so we will see that we are losing 30
Gold per turn.
Setting research, espionage and culture to 0%:
This means that all 100 of our
Commerce is turned into
Gold. Since we are not diverting any of our
Commerce to research, espionage or culture, we will generate no
Beakers, Espionage points, or "Extra" Culture (note: many buildings generate a fixed amount of culture per turn, like monuments, which is why I say "extra" culture)
Since we had 100
Commerce, we generate 100
Base Gold. Similarly to last time, it is
BASE Gold, because we still have to factor in our Gold-boosting buildings, namely our Markets and Grocers. In total they give a 50% bonus, so our 100
BASE Gold is converted into 150
Gold. Since our empire requires 30
Gold per turn in maintenance, our final
Gold per turn output is 120
Gold, with 0
Beakers per turn generated (except, oddly, the game always calculates a minimum of 1
Beaker per turn for some reason, so even at 0% you are still in fact producing 1
Beaker no matter what. Since this
Beaker comes out of thin air, it can make a very slight difference in the early game to know this).
Setting Reasearch to 50%, culture and espionage to 0%:
In this case, 50% of our 100
Commerce is devoted to research, and the other 50% is undeclared (which automatically means it is used to make
Gold).
50% of our 100
Commerce is 50
Commerce, which in turn become 50
BASE Beakers , which again is affected by your libraries, so you end up with 62.5
Beakers, which the game rounds down (not up!) to 62
Beakers. The remaining 50% of your
Commerce, again 50
Commerce, is converted to 50
BASE Gold. Since we get a 50% bonus on
Gold from our grocers and markets, we end up with 75
Gold per turn being made. There is still a maintenace cost of 30
Gold per turn though, so we only end up with 45
Gold per turn.
So, in summary of our 100
Commerce, 30
Gold Maintenance example:
100% research: 125
Beakers per turn, -30
Gold per turn from maintenance.
50% research: 62
Beakers per turn, +45
Gold per turn
0% research: 1
Beaker per turn, +120
Gold per turn.
And that's the difference between
Commerce and
Gold. Hope that makes sense!
symbol above the cities name just shows that the city is connected to the capital.
