To the end of the rule Population, append:

Each City has a publicly tracked Unemployed Population which is an integer that defaults to the City’s Population. Whenever the Population of a City increases or decreases, the Unemployed Population also changes by the same amount. If it would ever be set to a value above the City’s Population, it is instead set to the City’s Population. If it would ever be set to a value below 0, it instead becomes zero.

Create a subrule of Regions called Prosperity:

Each Region with one or more Cities has a Prosperity which is an integer that is allowed to be negative, and which is calculated as the sum of the Populations of Cities within the Region, subtracting twice the sum of the Unemployed Populations of Cities within the Region.