^ This. Doing the income like that makes for a fairly playable map. Now to get the 'GDP' for each individual city, you can do what the admins did. They looked on Wikipedia for the GDP for a whole country, then divided the population of the city by the population of the country and multiplied by the country's GDP. So for example:
GDP of USA= 15 Trillion
Population of USA= 311 Million
Population of New York = 8 Million
New York / USA = 2.57%
GDP of New York = 385.5 Billion
At which point you can scale as shown in Runway's topic. If you give me a list of all the cities you want 'GDP' for and what you want the highest income of a city to be, I could probably help you calculate the incomes. The highest income in the default map, Tokyo, is 559. New York is 365.
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"If in other sciences we are to arrive at certainty without doubt and truth without error, it behooves us to place the foundations of knowledge in mathematics."
-The Opus Major of Roger Bacon