That's not quite accurate. iOS exploits are worth a lot, bit OS X exploits are not worth as much. There is no relation between the two in terms of value. It's also much easier to write OS X exploits than it is to write Windows exploits in general. Windows has always been far ahead of OS X in terms of mitigations.

Safari on OS X is considered a soft target compared to the other browsers. It is the least difficult and has the fewest users. That is why the payout is less.