Everyone forgets that these books are all called guides , and they are not an exact science. For years Edmunds was what everyone used when they wanted to get a strong Insurance settlement , and they were notoriously high on their evaluations. Kelley Blue Book was often too low, but they went through a period where they were pretty reasonable. According to a reliable source, Kelley now uses auctions to figure the prices on Vipers, since there are so few out in the market compared to other makes ( figure an average year only has 1500 or so made ). NADA has been fairly vague, but seemed to be the first to try and break down Vipers more on condition than the others and this was years ago. All in all they are good for a basic background information, but in the case of the car you are looking at, none of the guides give credit for it being a blue/white 96/97. Since these model ( GTS )/years are often considered by many to be the iconic Viper example , they almost always go for more money. Finding one in good shape in the 20s in fairly rare, though it would be possible based on mileage. In all likelyhood , most 96 /97GTSs you will find will go in the 30s,40s, or 50s --- based on condition and mileage.
Everyone has their opinion, but a 11/12 year old car is very much going to be established by it's overall condition, paint, repairs, history, and after this many years there can be numerous variables.
Good Luck,
Bill Pemberton
Woodhouse