If you know your neighbors, you may want to ask them if they know recent sales prices in the building.
The NYTimes offers information regarding sale prices as does Street Easy. A good real estate search engine is quite helpful.
If you need assistance, feel free to call upon me-there is no charge for my services.
You'll need a Realtor w/knowledge of the area and your particluar building to do a comparative market analysis for you. Call Daniel Segal (212. 692-8327; firstname.lastname@example.org) manager of Prudential Douglas Elliman's East 42nd Street Office ( 205 East 42nd Street) for a referral.