Unless you've got a big load of nonobtainium, on Earth, it would be very costly or impractical or dangerous to keep a floating city in the clouds.
The easiest way to do it would be to have an enormous volume of lighter than air gas, such as Helium (expensive and rarer by the minute), or Hydrogen (dangerous and inconvenient (and the gazeous form needed for a floating city isn't even the same as the liquid form needed for rockets!)).
Of course, you've got the alternative of using solar panels (very heavy), and rotors, a lot of rotors (very heavy), and batteries, a lot of batteries (even more heavier) to keep a city in the air even at night.
Alternatively, you could have a nuclear power plant on the ground microwaving the energy to the city, but it should not go over the horizon.
In any case, a lot of cost, and you still have to pay to bring up to the city all the material and consumible you need to push a rocket from there. The only gain you have, is that you can use Oxygen from the atmosphere to lift the material there. But stopping at a cloud city is a little silly, since you lose any cinetic energy you had when you reached that altitude!
On the other hand, there are other planets where a cloud city would make much sense: Venus or Jupiter for example. There, you have an altitude where the pressure and temperature are similar to what we have on Earth surface, and where we could place a clould city exploiting the resources around.