On the plateau, Gato Loco certainly found that. His tests (see here) show on monarch level, increasing marginal cost per city until you get to ~30 cities, at which point, the marginal cost per city is 7 gold.
As for what Civ leader traits are important, I would not think Philosophical is as nice for this as Aggressive (Organized being a given). But it depends on style of play, warlike or peaceful. For the more peaceful way, Mao is the leader to pick. For aggressive, Tokugawa.
On higher play levels, being Spiritual might be more helpful, for kissing up to AIs. In which case, Asoka. Another possibility is Caesar, whose Expansive trait is mostly wasted, but getting Prats makes up for a lot.
As for what Civ leader traits are important, I would not think Philosophical is as nice for this as Aggressive (Organized being a given). But it depends on style of play, warlike or peaceful. For the more peaceful way, Mao is the leader to pick. For aggressive, Tokugawa.
On higher play levels, being Spiritual might be more helpful, for kissing up to AIs. In which case, Asoka. Another possibility is Caesar, whose Expansive trait is mostly wasted, but getting Prats makes up for a lot.