I recently had a look at this, and wanted to collate the info in a single post:

The most common neutral city income is 23 gold. I don't know how exactly it's distributed, but the lowest I've seen is 13, the highest 31. It's possible that cities with higher or lower income can be rolled. Currently my best guess is that it's something like 4d6+9 or 6d4+7.

Capital cities have higher income. The lowest I've seen is 17, the highest 36. If anyone has noticed incomes outside of these bands, please let me know.

For units, mercenaries and allies have 0 upkeep.
Regular units only pay 1/3 upkeep when in cities, rounded down to a minimum of 1.

So, in effect:

15 becomes 5 in cities. (Assassin)
12-14 becomes 4 (most Dragons, Archons)
9-13 becomes 3 ( Demons, Treants, Undead Beasts, both Pegasi, Slayer Knights, etc.)
6-8 becomes 2
1-5 becomes 1 (the vast majority of units)

Units with 16 (or higher?) upkeep, i.e. Silver Dragons, pay 0 upkeep, apparently due to a bug.
Summoned units pay full upkeep even in cities.

It seems to me that the Thief's 5 gold/city ability is vastly outclassed by other heroes' Engineering skill. However, it does stack on top of it, while only your best Engineering ability is used.

Between the higher income of capital cities, and the gold bonus from ports, fields and mines, an average of 25 gold per city seems to be a good number to work with, when considering how much unit upkeep you can afford. Overall, upkeep is pretty inconsequential when compared to the gold you make from looting, ruin treasure and questing, if necessary. Building up city defenses and buying new production adds up quickly, however.