Darvin: The extra trade route in every city doesn't hurt, either. I'd say it's as much the combination of Currency and Code of Laws that really causes everything to balance out. City upkeep is cut in half at about the same time as you get a whole bunch of wealth generation options, and that just opens up the game from that point onward.
The extra trade route helps too, especially if you happen to have some offshore islands. But the bigger benefits is the ability to sell cheap outdated techs for gold, and building Wealth. That is useful throughout the entire game, and can quite easily net you several hundred gold when you get Currency, and can start selling off Priesthood and other crap to all the AIs. Often means you can go 100% research for ~50 turns.
Courthouses can be good too, certainly. But be careful with them, and don't build them all over the place. They are very expensive builds, and in a small empire they are not worth to build. A rule of thumb I've seen bandied about is that once a courthouse would save you 4 gold per turn (so city maintenance is at least 8 gold per turn), you can start to *think* about it.
They become better in bigger empires, or in cities far from the capital (or Forbidden Palace or the relevant world wonder). And ofc, they are definitely needed if you plan to get corporations, because those put city maintenance costs through the roof.
It's what is both so good and sometimes a bit frustrating with this game, because so many things come down to "it depends" :D Sometimes doing X is a very good move, and sometimes it's a terrible move. There are very few hard and fast rules of thumb that are always good (or always bad for that matter).
Well, the RNG is always bad. Of course :D