You can try PropertyShark.com or Zillow.com. Also, try Googling the exact address in quotes. Often, real estate ads will come up that have a description and price for a particular unit(s). Sometimes, even the building itself is a source, with specifications for prices for various size units. You can also try the New York Times real estate ads online; if you specify just a street name, it is possible that the building is listed, with prices. This works, as well, for various real estate companies.