Just to clarify the specific scenario I am talking about: Say you work for your first 10 years of your life in East London and your gross salary from start to finish averages say, 240k/y finishing at R360k/y. Let's say you save 30% of your gross on average. This means that over 10 years you have R720k saved. Alternatively, you could have worked 10 years in Johannesburg, at an average of say, 480k/y finishing off at R720k/y, because the pay there is much higher. Lets say you save 25% on average. This works out to R1.2m of savings, which may be a little less than the alternative in terms of purchasing power in Johannesburg, however, at this point, you get to make a choice: if you like Jhb, stay, otherwise you can go to East London where R1.2m is worth a lot more,
and salary inertia could put you at R720k/y (+/- R100k). Similarly, you could move to Cape Town (or wherever) at a higher salary (although R1.2m in savings wouldn't necessarily buy you much more than Jhb

).
The same thing works on a global scale - work in London, New York, Silicon Valley, Singapore, Paris, etc. If you manage to make even a middle class income there, you are wealthy relative to the rest of the world, should you choose to live or retire there. The reverse is that you may be in the top 90% of East London earners, and after 30 years, you can't afford a decent house in Cape Town, or even the deposit for a decent house in San Francisco.