try kelly blue book or the black book or nada. it all depends on condition and mileage.... I have a 1980 Camaro that's worth around 60000
http://funadvice.com/r/15jg0n36uke <---- try first
I think NADA would be your best bet. They seem to give a price that are kinda high form my experience, but it should get you in the ball park. Kelly blue book gives more accurate prices IMO, but only goes back 20yrs, so to 1991:(.