Cars on line is the America's most popular online website for cars. I think it is most reputed website and more users there must be good price. We can compare price with same model & manufacture in the same site
that depends on the quality of the car and it's rarity. Some internet classified ads can have extremely cheap cars, but can be of poor quality. Always do research first.
The thing about websites like that one is that the prices will depend on the sellers. Fortunately they not only have already restored cars but also have cheaper project vehicle listed. So you may find a good deal on a car.