“A lot,” FBI chief James B. Comey Jr. said in response to that question at a technology conference in London, as quoted by the New York Times.
After a beat and a laugh, the official added that the amount was "more than I will make in the remainder of this job." With seven years and four months to go in his position and an annual salary of about $185,100, the Times extrapolates that, if Comey's logic is accurate, the Feds paid at least $1.35 million to have the phone unlocked. The FBI didn't confirm the number but just to be clear, they paid whatever enormous sum to find exactly no new evidence and no communications with ISIS on the phone.
Reuters also quotes Comey on the subject, from the same talk at the Aspen Security Forum in London. "In my view, worth it," he reportedly said, although it remains unclear to the public what's been gleaned from the phone of Syed Farook.
While it was originally believed to have been an Israeli mobile forensics firm that the FBI commissioned to unlock the phone, it's now unclear who was the recipient of that federal bounty.
Specifically, the hacker or hackers seem to have been able to exploit a security flaw unique to iPhone 5C models running iOS 9. There are, according to research from IHS Technology via Reuters, approximately 10 million 5C iPhones in the US, 84 percent of them running iOS 9. It's still unclear if the government plans to share the vulnerability with Apple, as it typically does in such instances. Apple, for one, has called for that disclosure.