The top 30 cities have 16% of the GDP by PPP of the world. 9780 billion out of 61000 billion in 2005. They had 4% of the population. 261 million out of 6.5 billion.
Price Waterhouse Coopers had a study of cities from 2005 to 2020
A 40 page pdf has some of the information with a focus on the UK and London
The top 100 cities have 25% fo the GDP of the world.
The top cities on the list are mainly from the United States (16). In 2020, there will still be 12 from the United States. Asia will move up the list.
the top six in 2005 being Tokyo, New York, Los Angeles, Chicago, Paris and London.
Five emerging economy cities are currently in the top 30 ranked by GDP (Mexico City, Buenos Aires, Sao Paolo, Moscow and Rio de Janeiro), but projections suggest that all of these cities except Rio will move up the global rankings by 2020 while fast-growing cities such as Shanghai, Mumbai, Istanbul, Metro Manila and Beijing will move into the global top 30 by 2020