It seems like when someone does a historical analysis of the wealth of these past tycoons, they often don't do a simple inflation calculation, they relate the wealth to the GDP of the US at the time. By that measure, both Rockefeller and Carnegie were quite a lot more wealthy than single-digit billions, though maybe not quite the same level as Elon Musk.

What makes Musk's wealth really incredible is how much of it is based on hot air (TSLA).