By Mark Wellborn

May 21, 2007 | 3:43 p.m

As The Observer reported first last week, a condo at the renovated Plaza Hotel has gone to contract for at least $50 million, making it the biggest New York City apartment sale ever. It joins an exclusive club of city properties, one with a membership fee of at least $30 million and one that opened just in the last few years:

4 East 75^th Street. J. Christopher Flowers bought the 20,100 square-foot Harkness Mansion in 2006 for $53 million.

15 Central Park West. Hedge fund manager Daniel Loeb purchased a 10-bedroom, 10,700-square-foot penthouse here in 2005 for $45 million.

834 Fifth Avenue. Rupert Murdoch purchased the 8,00- square-foot, three-story apartment in this fabled co-op building in 2004 for $44 million

Time Warner Center. British moneyman David Martinez bought both the 76^th and 77^th floors of the south tower, totaling 12,000 square feet, in 2003 for $42.25 million

1009 Fifth Avenue. Known as the Duke Semans mansion, cab driver-cum-Russian oil mogul Tamir Sapir bought this property in January 2006 for $40 million.

810 Fifth Avenue. Blackstone Group cofounder Pete Peterson bought David Geffen's 12-room duplex at 810 Fifth Avenue in May 2007 for $37.5 million.

2 East 63^rd Street. Russian businessman Lee Blavatnik purchased this 20,600-square-foot mansion for $31.25 million in 2005.