Depends what region you're buying it. 8", I would guess around $300+. I've seen a couple around 3-4" in the last year or so in Northern CA for about $100 and in the past around 8 years ago I bought one around 8" for $70, granted it was totally blind and one eye was completely flat and healed over and back then the normal asking price would have easily been $2-300 and what with the rising prices of NTT and even IT to some extent I would be ready to walk out with a much lighter wallet.
In my mind NGT inch for inch are really the most valuable datnoids these days with the obvious exception of ST.
That said if the price seems worth it to you it's not often you have a chance to get an NGT that size.