When Google reports its fourth-quarter earnings, one subject that is almost guaranteed to come up is the prospect that Apple could replace Google as the default search engine on Safari, the basic browser on all of its devices. The search contract between the companies is believed to be up for renewal this year. Given that Apple has spent much of the last few years trying to strip Google services like Maps and YouTube out of its products, the natural question is whether Apple will extend that to Google’s bread-and-butter search engine. Representatives from Google and Apple declined to comment on their search deal. One person who is commenting is Marissa Mayer, Yahoo’s chief executive. In a conference call with analysts, Mayer said Yahoo would seek to replace Google as the default search engine on Apple’s Safari browser. “Safari users are the most engaged and lucrative in the world,” she said. Mayer, who built her reputation overseeing Google’s internet search business, said that Yahoo remained committed to search. The company is discussing changes to its 10-year search partnership with Microsoft, which is at the midway mark. So let’s say that Apple dumps Google in favor of another search engine like Yahoo or Microsoft’s Bing. How bad would that be for Google? According to various reports, the impact on Google’s bottom line would be minimal. There’s even an argument that Google could come out ahead. Central to these analyses are two questions that are difficult to answer. The first: How much does Google pay Apple to be the default search engine on iPhones, iPadsand Macs? Estimates are all over the place, from 35% to 80% of the revenue made for searches performed via the Google search bar.