15b USD/year / 500 m users = 30 USD/user/year
3 searches/user/day = 1000 searches/user/year
30 USD/user/year / 1000 searches/user/year = 0.03 USD/search
AFAIK, 0.03 USD per search (or impression) is in the right ballpark for advertising.
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Data used (retrieved from Google) 446m Safari users. An average user searches on Google 3 - 4 times per day.