Uber's valuation jumped $15 billion in a single week, according to one billionaire hedge fund manager

Uber is one of the most valuable private tech companies in the world. It's raised $5.9 billion at a $41 billion valuation, and a new round of funding -  an additional $1.5 billion to $2 billion - would make Uber the most highly valued private tech company of all time, at over $50 billion.

Mike Novogratz, the president of $70 billion investment fund Fortress Investments, says that back when Uber was starting its roadshow to raise a recent round of funding, its valuation jumped $15 billion in just a week.

On Sunday's episode of "Wall Street Week," Novogratz said he'd had an "interesting meeting" with former Uber CFO Brent Callinicos, who Novogratz referred to only as "Uber's CFO" during his chat.

"Uber's the fastest growing company, maybe in the history of the planet. And it's a brilliant idea, and they were raising capital," Novogratz said. "They had started their roadshow at a $25 billion valuation. And a week later the valuation had jumped to $40 billion."

"Wall Street Week" host Anthony Scaramucci interrupted Novogratz to ask him: "Did the valuation go up $15 billion just because they went on the roadshow and told the story?"

"Yes," Novogratz said.

Novogratz asked Uber's CFO how the company could justify such an insane valuation. Novogratz says Callinicos told him Uber takes between 20% and 25% of the fare drivers collect, but that in the future, Uber could hike that up to 25% or 30%. "If you look at the growth and you add that extra pure margin, we'd make a lot more money and we justify this $40 billion valuation," Novogratz said, recalling what Callinicos told him.

Earlier this year, Novogratz and Fortress Investments backed Lyft, Uber's biggest US-based competitor.

You can watch the full episode below. Tune in around minute 11 for Novogratz talking about Uber.