Apple did not add ddg in the past few years despite people asking them to, which might indicate that Apple, too, asks for money for these referrals.
So this begs the question - how how much did ddg have to pay?
Mozilla grossed 311M for FY2012, almost all of which came from their deal with google: https://static.mozilla.com/moco/en-US/pdf/Mozilla_Audited_Fi...
A couple of years ago, I was at a ddg hackathon when inclusion came up. I believe they mentioned that it was on the order of a 7 digit number to be include in the list of search engine. Regardless of how much it was, they couldn't justify spending that much money on being include in Apple devices.