Prices for classic cars are usually highly negotiable. Classic cars are in high demand, and as time passes, the supply becomes more limited. Because classic cars are so rare and valuable, the seller typically has a great deal of negotiating power over the price of their car.
Numerous factors can affect the price of a classic car, most notably how well the car has been maintained and how many original parts it has. The NADA Guide is a great place to get an idea of what a specific make and model of car currently sells for, but because classic cars are so rare, the final negotiated price can differ considerably form the NADA recommendations.