An iPhone costs roughly the same no matter where you live, it’s still more fun to visit Vietnam on an SF salary then it is to visit SF on a Vietnamese salary.

And for the most part when the prices differ 9 out of 10 times it's more expensive outside the US (and sometimes significantly so).

Whenever I go to the US it's shopping time. Pretty much everything is cheaper. And many items you just can't get elsewhere (which is another aspect many ignore).

I know this is going to be controversial, but healthcare-wise high earners might be better-off in the US too. Generally it would seem to me, the more money you make the better US is in comparison.

agreed. But that's a fundamentally different concept you are describing. The point is you cannot compare $100 salary and living in A with $100 salary and living in B unless you know the values of A and B. This definitely holds for "modern" western countries where social welfare/tax systems/costs of living/healthcare have a huge impact on living standards and hence what a salary of value X means. Generally speaking you might say more money == more fun, but I don't know of any data to back up that claim once we are talking about some dollar amount above the poverty limit.