Sorry to lecture, but there is no such thing as a “Media Composer Symphony”. There is an “Avid Media Composer”, and there is an “Avid Symphony”. If it is a Media Composer, it cannot be a Nitris, since Media Composer only works standalone, with Mojo, Mojo SDI, Adrenaline or Adrenaline HD (or, since v3 with Mojo DX or Nitris DX). If it is a Symphony, it cannot be version 2.8, since there was no Symphony Nitris 2.8, they went from 1.8 to 3.0.
Whether it is one or the other will greatly affect the value, IMO.
Also, it would be good to specify the CPU included, and whether or not you are including storage, monitors, etc.
Currently a new Symphony Nitris DX system, turnkey, lists for something like 36K. A new MC with Nitris DX for 24K. You can look at sites like Ebay to see what prices are being asked for used systems.