Immortal what?
Total cost for having all generic buildings (not coastal, no special power plants, no resource buildings like forge or monastery) in a city is around 60 GPT per city, add 6 GPT for a railroad connection. Please do show me that city which can produce 66 GPT without having 2 puppets to support it. That's excluding things like military support, paying off city-states, research agreements, unit upgrades...
Not all playstyles aim for massive conquests which are the only way to support mega-cities, so stop talking nonsense please. Maybe you'll learn some downright useless information.