The estimated $948 million price tag for a new retractable roof stadium for the Atlanta Falcons is a pipe dream, experts say.

Two professors who study stadium deals told the AJC those estimates are generally low and the costs for the new stadium will probably exceed $1 billion.

 “One of the real rules in these analyses is they always underestimate cost,” said J.C. Bradbury, chairman of the Health, Physical Education, and Sport Science department at Kennesaw State University.

“When they say it will cost $900 million, I’m thinking, ‘How much over a billion is it really going to be? ‘”

Wednesday the Falcons and officials with the Georgia World Congress Center announced plans to build a $947.7 million stadium with a retractable roof either on a GWCC-owned truck parking lot just north of the Georgia Dome, or at an unspecified site south of the field.

Rick Burton, a professor of sports management Syracuse University, told the AJC cost overruns of 5 percent to 10 percent are common.

“It sounds to me like they are trying to price point this so they can say that they can do this for just under $1 billion,” said Burton.

Falcons president Rich McKay said team officials, who previously favored an open-air stadium, now believe a retractable-roof facility is the best solution for all parties. The new stadium would accommodate both indoor and outdoor events.

The Falcons have rejected plans for a renovated Georgia Dome or adding a retractable roof to it, pointing out that 22 teams have moved into new or completely rebuilt stadiums since the dome opened in 1992.

McKay and GWCC Authority executive director Frank Poe say much work remains to be done to reach a deal, but said progress has been made.

Poe said the state contribution to a new stadium would be capped at the estimated $300 million covered by the hotel-motel tax extension passed by the Georgia Legislature in 2010.

NFL commissioner Roger Goodell, during a visit last year, made it clear a new stadium would be needed if Atlanta wants to land another Super Bowl. The Georgia Dome hosted the championship game in 1994 and 2000, but two bids to bring the game back to the city have been rejected.

The Georgia World Congress Center Authority runs the current stadium as well as the massive convention center and nearby Centennial Olympic Park.