I'm looking around for some figures to base this on.
I could use per capita GDP [1] but I'm not sure GDP is all that good a measure of how much spare cash an average person has so I'd be interested in any alternatives.
I'd be grateful for any ideas.
[1] https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(PPP)_per_capita#Table
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EDIT: the text "... but I'm not sure GDP ..." originally read "... but I'm sure GDP ...", this was a typo.