At about $9 million annually, it's a significant departure from Nutter's study, done by economists at Boston College and released last summer, which put the cost at $258 million, or $22.3 million annually.
Bolton's findings are more closely aligned with a Board of Pensions actuarial report in 2003 that estimated the annual cost at $7 million.
Councilman Darrell L. Clarke said Thursday that he could not verify the $100 million figure, but that the difference would be "significant."
Bolton Partners declined to comment.
Anthony Webb, author of the Boston College report, referred questions to George Lausch, an editor at the college's Center for Retirement Research, but Lausch did not return calls or an e-mail seeking comment.
Mark McDonald, a spokesman for Mayor Nutter, said that the mayor's and City Council's studies were "both credible reports," and that even at $100 million, DROP has to go. He would not discuss the apparent discrepancies that led to two different numbers.
"The city faces a series of very serious issues related to the pension fund and improving its level of funding," McDonald said. "If, a year ago, we had announced that the DROP cost the pension fund $100 million, we would have concluded it was too expensive for taxpayers. Whether the number is $100 million or $258 million, DROP must be eliminated."
The question is whether Council members - seven of whom have either already benefited or stand to benefit from the program - and the mayor will agree that the lesser number is valid.
Bolton apparently convinced Boston College to consider factors it had not in the original report, which came out in August. Sources would not say what the discrepancies were. Although Bolton has arrived at an estimate of the cost of DROP, the company has not yet completed its full report, sources said.
