Civilian unit cost curve

[MiniatureDeathRobot]

I understand that civilian units increase in cost as you build more of them, but are the exact rules governing this documented anywhere? Is there a general rule or every unit slightly different? Does purchasing a unit with gold increase its cost in terms of production, or does it only increase the gold cost? It feels like it would be really useful to know all this for planning and deciding, say, whether that use of a Builder charge is really worth it or whatever.

 

[ezzlar]

https://forums.civfanatics.com/threads/formula-thread.600534/