"Salary can't and shouldn't be compared accross the globe."
I disagree. If the hiring company is looking for the best candidates in the world, then they must be prepared to shell out comparable 'global' salaries.
At a "unique/very few in the world", yes, location only makes your (high) salary goes higher. But then you have 6 digits in your salary regardless if it's in $/€/£
At a "specialized position" (I guess several people on HN are here), you absolutely don't need to offer the same salary regardless of geographical position, but it has to be competitive.
The talent war is continental rather than global. There are plenty of VISA issues and legal troubles that make it impossible for a lot of people to work in a lot of countries [and for those who do, it's preventing them to get the top end of local salaries].
Thus there is no global salary.