Cheap is relative, but iOS exploits used to be the most expensive at Zerodium. That's no longer the case, Android zero-click exploits are the king of the hill. Arstech has a pretty interesting article on this change.

The most interesting aspect is how exploits are chained together to achieve the zero-click malware. The two primary suspects for this in iOS are Safari and iMessage application.

No, I won't switch to Android devices; when all aspects of the devices are taken in to account, I believe that the overall security of the iDevices is still better, than on the aDevices...