It would be very difficult for one self-made man to amass a fortune equal to 0.60% of the entire US GDP. Today, that would be about $96 billion, so Bill Gates at $66 billion got close, but no cigar.
As the Koch Brothers are fond of saying, “We made our money the old fashioned way, we inherited it.” If anyone is going to reach north of $100 billion today, they’d probably have to have a running start with many billions in the bank at birth. And most people in that situation, the Walton family, for instance; have no stomach for the heavy risks of the entrepreneurial arena. They invest most of their money in safe tax shelters off shore, and give a few billion here and there to right-wing causes in hopes of obtaining more welfare for the rich and keep lots more of what they didn’t earn.
The Rothschild family in Europe is worth $1 trillion, so even in that tax environment, families that stick with the family business and build wealth over many generations can accumulate enough to live comfortably for the rest of their lies, even if they should happen to live for a billion years with no income.
That said, I have to question why you are asking this. The implication seems to be that the US would be far better served if a handful of robber barons could build trusts and monopolies and accumulate nearly all the wealth of the nation. Are you suggesting that nothing short of such wealth disparity is sufficient to motivate someone to innovate? If so, I would say that society appears much better served by policies that spread entrepreneurial spirit across a far greater spectrum. Our consumer driven economy works far better with a healthy, well-heeled middle class and millions of entrepreneurs lending their innovation to it.