I'm having some trouble understanding the value of cities. In my first game using commerce, when buying a city, when I go to build it says something like "1 gold for first 10 armiea, 2 gold for 20 armies, etc..."

I built a couple cities and it looks like it is actually taking away from my gold.

Clearly I'm not understanding the calculation.

Separately, what other advantage is there to building a city (if any)?

Thanks in advance.

What you quote was the cost of buying armies, not cities.

Cities cost gold to build and give you 1 gold per turn. So they take gold in the short term but pay off in the long run.