I think the idea is you should have to spend significantly to Get large.
If settlers cost 300 hammers (+100 for each additional city you have)
Then REX will not be a good strategy, even if that settler gave you a bonus with no happy/science penalties
You could also make cities slow to develop (every population point costs 80 food)...so it might be worthwhile building up the capital rather than expanding.
Then to ensure the map doesn't stay empty, later techs/policies can make settlers cheaper/start with more pop/buildings.
An increase of settler costs by +100 per settler/city will make settlers soon be unrealistically more expensive than a modern day world wonder ... some Civ players like to build 10 - 20 - 40 - 100 cities during one game ... should those players better not play Civ6?
New cities in Civ5 usually cost a lot to get them running unless you want them to be useless for some hundred turns (marathon speed) ... buy monument, buy granary, buy aqueduct, buy colosseum, buy workshop, buy library, ...
Civ is a game about old empires (with exception of USA which are only a few hundred years old) ... so most cities should be placed in the early eras (ancient, classic, medieval) ... you can still later conquer, raze, rebuild cities ...
The opportunity costs for a city are usually the costs for the settler plus military units to guard it plus all the buildings you buy in the city. Your return is the amount of production, science, gold it produces, which highly depends on the resources (food, production) at the city's location.
In Civ Call to Power, the empire size was limited by government, allowing I think 10 cities at start (the 11th city would cause a -10% penalty on everything or so) and later went up to 60 or 80 cities (Trade-Union) for the end-game ... The problem here is the global penalty ... the -10% costs you much more production, science from your core cities than you can compensate with the new city unless it contains some luxuries or strategic resources which allow to boost all cities ...
Civ games usually follow the line : found a city, grow until you reach the local happiness cap, expand, research to increase happiness cap, grow, expand, research, ... it is a very organic process, like growing weed ... (Civ5 with global happiness was an exception since exhausting your global happiness made it difficult to found/conquer more cities ...)
There were tries to implement Empire-Stability and administrative costs for wide empires in Civ-Games / mods ...
Example : You could add an administrative department Level I in your capital with 3 specialist slots ... allowing up to 9 extra cities to be managed. When you expand beyond, You need to upgrade administrative department to Level II in capital and add administrative department Level I in some of the cities until all your cities are managed. Costs would be similar to national wonders of respective era ... If a city would not be managed, it would be like a puppet in Civ5, generating some Gold but no Science or Culture ...
In general the best solution to the expansion discussion is :
Players who don't like expansion should play on small to standard map size, players who like expansion should play on large, huge or giant without restrictions ...