To quote the 4Q12 report:
"Opera’s monetization strategy for its desktop browser revolves
predominantly around search. Google is Opera’s global search
partner and provides the majority of desktop monetization. This
global partnership is supplemented by local search partnerships
in certain markets, such as Russia, Japan, and China, where
Opera works with Yandex, Yahoo! Japan and Baidu, respective-
ly. In addition, Opera has signed up e-commerce players such
as Amazon.com (USA, Germany, Japan), Booking.com (64
countries), and Ozon (Russia) to further enhance ARPU."
"Desktop Consumer" made 16MUSD in the last quarter: that's ~64MUSD annually. Of course, that won't be all the Google revenue (as there's also the default browser on other platforms), but gives a reasonable baseline figure.