I found some information in \Sid Meier's Civilization VI\Base\Assets\Gameplay\Data\Districts.xml
For most of districts configuration looks like:
Cost="60"
CostProgressionModel="COST_PROGRESSION_NUM_UNDER_AVG_PLUS_TECH"
CostProgressionParam1="25"
For Aqueduct, Neightbourhood:
Cost="60"
CostProgressionModel="COST_PROGRESSION_GAME_PROGRESS"
CostProgressionParam1="1000"
For spaceport, city center and wonders there is no CostProgressionModel configured.
It look like cost formula may be connected to game progress/number of technologies/number of districts.
I have no idea about exact formula yet. I need to gather some more test data during gameplay
For workers and settlers there are also some parameters in Units.xml:
Settler:
Cost="80"
CostProgressionModel="COST_PROGRESSION_PREVIOUS_COPIES"
CostProgressionParam1="20"
Worker:
Cost="50"
CostProgressionModel="COST_PROGRESSION_PREVIOUS_COPIES"
CostProgressionParam1="4"
On standard speed cost of settler goes like: 80, 100, 120...
It looks like cost formula of these units, is connected only to the number of specific units already produced: Cost + CostProgressionParam1 * number of unit already produced.