City costs have two components, the first being based on the total number of cities you have and the second based on the city's distance from your capital, Forbidden Palace or Versailles - whichever is nearest. Only the second component can be reduced by building a Courthouse, I find. To see the breakdown, just mouse over the 'gold' icon, top left of City Screen: to see breakdown of your total costs, mouse over the icons in the Financial Advisor screen.
In fact, running your mouse over practicall any icon will reveal something. Sometimes a click is also needed, but be careful this doesn't send an overlooked selected unit to its death.