“They are actually good investments, in the sense that the current business model for all major sports is a license to print money,” said Victor Matheson, a professor and sports economist at College of the Holy Cross in Massachusetts. “It’s hard to lose money in sports, the NFL in particular.”

The NFL is a special case compared to the other leagues, and not just because team valuations are far higher. “I’d rather be a middle-market NFL owner than MLB owner because of the way media revenue is shared,” Sunkin said.

“Cities’ willingness to spend hundreds of millions of dollars on pleasure palaces for sports franchises helps prop up the value of sports franchises,” said Michael Leeds, a professor and sports economist at Temple University in Philadelphia. “I would say this goes back as far as the 1950s, when baseball started moving West.”

Ever-escalating national television deals are another big driver behind surging sports team values — particularly in football and basketball.

“Media, advertising, sponsorships, those dollars are very strong. It’s one of the few entertainment concepts people watch live,” Sunkin said. “Advertisers know who’s watching, and that translates to major rights deals.”