There is medicare for old people in the USA, as imperfect as it is. Also developing countries might not be nice places to raise your kids unless you do the private school / live in a bubble thing.
Try replacing the developing world with 'the US ghetto' or 'rural usa'. You can approximate the costs fairly closely in some of those cases. $40k/$30k houses in small town upstate new york or texas for example and %10-%15 federal tax rates, still be within domestic flying distance of family.
All valid points, but I still doubt you can get the same range of lifestyles (eg. car free, beach proximity, unmatched availability of foodstuffs) that cheaply, internal to the US, within a decent climatic zone, somewhere you'd actually want to live. Besides, you're neglecting the linguistic/cultural stimulation that comes from being somewhere truly new.
Try replacing the developing world with 'the US ghetto' or 'rural usa'. You can approximate the costs fairly closely in some of those cases. $40k/$30k houses in small town upstate new york or texas for example and %10-%15 federal tax rates, still be within domestic flying distance of family.