The best move would be to Contact the managing agent and a local agent in your area. The managing agent will have all sales that are closed. The Realtor might should have access to places that are pending closing as long as they are listed with a Realtor on the MLS (Multiple Listing Service).

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.

This information is also in the hands of the managing agent for the co-op.

I don't know if that information is available on Trulia. Alternatively, you can contact a Realtor (like me), you can conduct a search on ACRIS or you can pay for the information from Streeteasy.