NEW YORK CITY-A partnership led by Related Cos. is under contract to sell the 265-unit Sagamore rental apartment building on Manhattan’s Upper West Side for $140 million, according to a source familiar with the deal. A spokeswoman for Related declines to comment on a Bloomberg report of the sale, which says the buyer was a unit of Chicago-based LaSalle Investment Management. LaSalle’s spokeswoman tells GlobeSt.com the firm does not disclose details of its acquisitions.

The $140-million sale price works out to more than $528,000 per unit. It’s the largest multifamily sale in Manhattan since Equity Residential closed on its $142.7-million purchase of the 293-unit Longacre House at 305 W. 50th St. from Macklowe Properties in March 2010, according to Real Capital Analytics data.

Equity’s Longacre buy followed its acquisitions of two other Macklowe residential assets: the 323-unit RiverTower at 420 E. 54th St. for $181.5 million and the 294-unit 777 Sixth Ave. for $150.8 million. Both sales closed in January of last year.

In the interim, a joint venture of Angelo, Gordon & Co. and Extell Development Co. closed on the acquisition of the 157-unit Helmsley Carlton House from the Helmsley estate for $166 million, a sales price that equated to more than $1 million per unit, according to RCA. However, the Helmsley Carlton at 680 Madison Ave., built in the 1950s as a strictly residential property, had been operated in the interim as a hotel-apartment building specializing in long-term rentals. Extell reportedly plans to convert the upper floors into condominium units.

Bloomberg reported that the Sagamore sale generated interest from at least 15 bidders. Doug Harmon, managing director of Eastdil Secured, represented Related and its joint venture partner for the Sagamore, the State Teachers Retirement System of Ohio, according to Bloomberg. Related EVP Bruce Beal Jr. says in a statement that the sale of the Sagamore represented “an opportunity to both extract value for Related and our partners and to reinvest the capital into our acquisition and development initiatives.”

Built in 1998, the 20-story Sagamore at 189 W. 89th St. is reportedly among the few luxury rentals in the Upper West Side submarket, which is dominated by cooperatives. It’s an 80/20 property, meaning that 20% of the units are set aside for low-income tenants. For market-rate tenants, units start at $2,595 per month for a studio, according to Related’s website for the property.

RCA says 137 Manhattan multifamily properties worth at least $5 million changed hands last year, for a total of more than $2.2 billion. That made Manhattan the largest apartment investment sales market in the US during ’10, and at an average sale price of $287,405 per unit, also the priciest. New York City’s outer boroughs contributed about another $1.3 billion from 173 transactions, according to RCA.

Cap rates across the multifamily sector in Manhattan averaged 6.1% in ’10, a number which RCA data show as higher than the average caps for apartment sales in Washington, DC, Los Angeles, San Francisco or Charlotte. Massey Knakal Realty Services says the average cap rate for elevator apartment building sales on the island last year averaged 4.84%, up 174 basis points from a low of 3.1% in 2006.