I'm very familiar with websites showing that if you make $30k you're in the top percentile worldwide. The big problem with these "comparisons" is that they are blind to the cost of living differences, as well as the accepted standard of living in different nations.

If you live in a nomadic society where all you need is a sturdy tent and some sheep and goats, your cost of living is vastly lower than if you live in NYC, for example, so comparing the net worth of people from those two locations is ridiculous.

However, having said all that, it's true that even after considering these differences, the middle class in a Western democracy lives a far more luxurious life than much of the upper class in third-world countries.