Google Chrome is the most popular browser on the planet, so there's no question that it has Google Search set by default. However, what about the Safari browser in iOS? Apple might only hold around 15 percent of the global market with regards to smartphone operating systems, but that still represents hundreds of millions of devices. Google wants its search engine primely positioned as the default in the iOS version of Safari, and over the years it has paid dearly for that distinction.
In 2014, it's said that Google paid $1 billion to Apple to remain the default search engine in iOS. Last year, that figure reportedly tripled to $3 billion. To renew its contract for 2018, it's said that the sum will triple yet again to an astounding $9 billion according to Goldman Sachs analyst Rod Hall. If that wasn't enough, it's said that Google's payout to Apple could swell again in 2019 to $12 billion.
It is likely that the reported $9 billion figure takes into account default search engine status in iOS Safari along with results delivered by Apple's global Spotlight search and results provided by Siri.
It's definitely a delicate balance that Apple and Google have to play with regards to their search arrangement. Google is willing to pay the hefty sum in order to gain access to Apple's massive and incredibly loyal install base, and the return on that investment is likely far greater thanks to the search revenue generated by those users.
As for Apple, it can turn the screws on Google for what amounts to "easy money", but if the company gets too greedy, customers will see their default results taken over by providers that often don't provide as comprehensive results. Considering that most people won't change their default search engine (despite the fact that it's easy to do), Apple would likely get an earful from its customers.