In light of the recent Panama Papers scandal, it is worth considering how much revenue is actually hidden away in tax havens. The most widely cited estimates seem to be from a paper by Zucman (2014; Journal of Economic Perspectives, Table 1). These estimates are cited in a recent report by Oxfam (p. 12), entitled 'Ending the Era of Tax Havens: Why the UK Government Must Lead the Way'. And very similar figures are offered in a recent report by the Tax Justice Network (p. 42), entitled 'The Price of Offshore Revisited'.
Zucman reports revenue losses from offshore wealth of: $75 billion for Europe; $36 billion for the United States; and $190 billion for the world. Using GDP data from the World Bank, and tax revenues as a percentage of GDP from the Heritage Foundation, I calculated that these estimates represent losses of: 1% of revenues for Europe (0.4% of GDP); 0.8% of revenues for the United States (0.2% of GDP); and 1% of revenues for the world (0.2% of GDP). (Tim Worstall obtains similar figures here.)
It should be noted that these estimates are most likely conservative. As Zucman writes:
My method probably delivers a lower bound, in part because it only captures financial wealth and disregards real assets. After all, high-net-worth individuals can stash works of art, jewelry, and gold in “freeports,” warehouses that serve as repositories for valuables—Geneva, Luxembourg, and Singapore all have them. High-net-worth individuals also own real estate in foreign countries.
For comparison, tax revenues in the UK decreased by 4.5% between 2007 and 2009, over the course of the financial crisis.