"It could be in the millions," said Budget Director Stephen Agostini. "Exactly how much, I'm not sure yet."
Still, attorney David Rudovsky, who has a pending federal suit over prison overcrowding, said that the city has more work to do.
"I do give the mayor and the courts and the district attorney some credit over the last year for reducing the population by about a thousand," Rudovsky said.
"But 8,600 is still too high. They are still triple-celling somewhere between 1,500 and 2,000 inmates. We think any triple-celling is impermissible."
Triple-celling is the controversial practice of putting three inmates in a cell designed for two. Deputy Mayor for Public Safety Everett Gillison said that the number of triple cells has been reduced.
Gillison said that his goal is to get the prison census down to 7,600.
The city houses convicted prisoners and inmates facing trial in six main correctional facilities. Officials said that the population has declined for two key reasons.
The first is that about 250 inmates who are the responsibility of the state correctional system have been transferred out.
The second is that the city has found ways to more efficiently move non-violent offenders through the legal system.
Prisoners facing multiple charges now come before one judge instead of visiting several courtrooms, which can reduce their length of stay.
Some paperwork is being processed electronically and a program to do hearings via video conferencing is being expanded.
Prisons Commissioner Louis Giorla said that better management and coordination among police, courts and prisons - who work together on the Criminal Justice Advisory Board - were key to making those changes.
"In the past, the way the prison population was reduced was through emergency releases or court intervention or some kind of reaction that doesn't address the efficiencies of the system," Giorla said.
In a 2007 study, the Pennsylvania Intergovernmental Cooperation Authority said that the growth in the city prison population was not due simply to a spike in arrests, but also because inmates were staying in prison longer, in part due to delays and complications in court hearings.
Gillison said that he hoped to continue the decline this year. He said that the city is looking at more technology improvements and is considering day-reporting centers for inmates granted early release or for people held on bail.
"Do we have another 500 to 1,000 people who might be able to take advantage of that? I believe we do," Gillison said.
Agostini said that he expected the population drop would translate into savings this year in the prisons budget, which has ballooned over the past decade.
In the 1999 fiscal year, the prison budget was just $112 million. In the current fiscal year, it stands at $249 million. According to PICA, the dramatic increase in the population over the years led to the spike in costs, requiring more money for health care, facilities and correctional staff.
Agostini said that it was too early to say exactly how much the recent decline would save the city in the current fiscal year, but several key savings have already been made.
As a result of the reduced population, the city has cancelled contracts to house some prisoners in Monmouth and Passaic counties in New Jersey. In the previous fiscal year, the city spent $4.7 million on those contracts.
The city has also stopped using the gymnasium at the Holmesburg prison to house prisoners. That prison was closed in 1995, but in 2006 the city outfitted the gym with beds for 80 prisoners. Later the space was updated to accommodate 100. That practice ceased in October, saving the city $56,000 a week, according to prisons spokesman Robert Eskind.