It isn't just one study, it's what I've gathered over the past 3 years. Some input is from clients I work with who run PPC, some input is from conference presentations I've attended, some input is from paid, proprietary studies, some is from trade magazines, and so on.
You can find some information like this through a Google search, but it usually comes out in this same general "ballpark", and it's never been different by any sigificant degree.
I've even read some reports that say organic generates 90% when both are in place, but I think that must either be an unusual market niche or perhaps a misstatement. Still, who knows when there are so many variables involved -- the creative used in the PPC ad, the title and snippet in the organic listing, the market demographics, whether a OneBox is on the SERP, etc.
So the exact ratio will vary for each particular situation, but it will almost always be heavily weighted toward organic clicks.