Poor countries are cheaper to life in than wealthy ones (GDP PPP).
Also countryside is cheaper than cities.
It should be affected by GDP, inflation and urbanization.
It would affect income of Poor, Middle Class and Wealthy population.
This way inflation would effectively lower income of all people.
Also this would simulate generally higher prices in more developed countries and in cities too.
Can private sectors raise their reduction of disposable income over time?
It could depend on several things like low wages, lack or poorly funded public services and so on.