SAC has about 70,000 square feet spread between the second, third, fourth, fifth and sixth floors. The firm took a 10-year lease for the space in 2011 and was one of the first tenants in the building, which features a pool, gym, floor-to-ceiling windows and 10-foot ceilings. SAC paid between $80 and $99 a square foot, according to people familiar with the situation.
Based on recent leases, SAC could get between $100 and $140 a square foot for a sublease, according to a broker with knowledge of the market. Other tenants in 510 Madison include hedge fund firms Senator Investment Group, Valinor Management, Junto Capital Partners and Jay Goldman & Co.
"SAC can salvage value from its existing office space as it is located in two premier buildings," said Jonathan Luttwak, co-head of global alternative investment services at real estate broker Cushman & Wakefield.
Luttwak said SAC would almost certainly get more than it paid.
"Not surprisingly, SAC also signed for their existing space at relatively low points in the market and is paying low rents for those buildings," he said.
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SAC's headquarters on the Stamford waterfront would fetch far less.
The firm's main offices at 72 Cummings Point Road--which features the large trading floor that Cohen himself sits in the middle of--would get about $35 a square foot to rent, according to a person with knowledge of the local market. A second building nearby at 22 Gatehouse Road would likely get less, about $30 a square foot.
"Stamford in general is a sought-after location for financial firms and it's basically as acceptable as Greenwich for hedge funds," said Steve Greenbush, a Stamford-based broker with CBRE. "At the same time, we've seen a slowdown in demand since the financial crisis so the market isn't quite as hot as it once was."
SAC would have some high-powered help were it to downsize. Its primary broker in New York is Newmark Grubb Knight Frank vice chairman Neil Goldmacher, widely considered a dean of Manhattan hedge fund real estate.
In Connecticut, SAC has recently been represented by Brian Carcaterra of CBRE. Carcaterra worked at Newmark from 2003 to September 2012 and is close to Cohen's firm through a personal relationship with soon-to-depart Chief Operating Officer Sol Kumin.
Carcaterra and Kumin both played on the same lacrosse team as undergraduates at Johns Hopkins in the late 1990s.
The two men initially took different paths after college. Kumin stayed in finance and joined fast-growing SAC early in his career. He rose quickly and ultimately "helped create our global strategy and footprint," as Cohen put it in a November 21 note to SAC employees. Carcaterra, by contrast, played professional lacrosse from 2000 to 2003 and was a Major League Lacrosse All-Star and champion, according to his CBRE biography.
Goldmacher and Carcaterra didn't respond to requests for comment.
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Potential prices for SAC's key offices abroad vary. The average rent for "Grade A" office space in London's City neighborhood range from $71 to $89 a square foot, according to a third-quarter report from Cushman & Wakefield. Average luxury rents in Hong Kong are around $103 a square foot, according to the same report.
—By CNBC's Lawrence Delevingne. Follow him on Twitter @ldelevingne.