Here’s a fascinating article on the math behind incremental games; Numbers Getting Bigger: The Design and Math of Incremental Games, by Alexander King.
You know what it all boils down to? It’s this handy formula:
Price = Base Cost × Multiplier(#Owned)
It works particularly well, psychologically, if the multiplier is set between 1.07 and 1.15. Less than that and you progress too quickly to enjoy each new upgrade, and much larger than that and the game bogs down and feels like a tedious grind. Somewhere between 1.07 and 1.15 is the sweet spot, used by many, many idle/incremental games.
And me, too.