[stanleydrew]

This is all true but doesn't invalidate the point unless the rich and poor are spending the same proportion of wealth/income on these various categories.

The fact that the rich can afford to save more of their income proves that isn't true.

A more pedestrian proof would be that even if I make 10 times as much money as someone else, the most expensive gallon of milk I can buy is still only double or triple what a poor person would pay.