Expanding has no meaningful cost. Expansion is inherently self-supporting and self-rewarding. Most of the time, the only real consideration is running out of spots which can afford a city enough production to be worth the effort, and perhaps having a governor to spare until population reaches a self-sustaining threshold.Not really, it all depends on available space, available troops to defend your city, loyalty, population and whether or not you should spend your production on something else. Expanding has a cost, and if you just spam settler after settler you'll just end up funding your neighbours' expansion instead.
All other supposed costs are easily mitigated in the majority of situations, and therefore canards in such a discussion.