US intelligence agencies pay hackers as much as $250,000 for iOS security exploits or $200,000 for a Chrome or Explorer vulnerability. The agencies will not tell you about it: they would use that exploit to gain access to people's devices.
These secret backdoors that nobody knows about are called zero-day exploits. When hackers find one they can contact a person called The Grugq, a security researcher based in Bangkok. He will sell the secret to the highest bidder among all interested intelligence agencies in the world. His latest and biggest sale was $250,000 for a iOS security hole, paid by an undisclosed "US government contractor."
The iOS exploits, says the Grugq, are the most expensive because they are very rare thanks to the stronger security of Apple's iOS compared to something like Android. The latter's security holes go for about $30,000 to $60,000. [Forbes]