I don't think that's hard at all
Now tell me, what defines a player's city?
Is it some kind of townhall that the player posesses?
If so, it should be fairly simple: Create a continious periodic timer event for each 60 seconds.
Check how many units of a specified type the player has and add the income to the player based on the amount of units of the specified type.
There is not much more to it then that.
You can ofcourse use your imagination and expend it to your needs.
Tell me if you need an example or if the definition of a "city" is not a simple unit being posessed.