In the days of analog displays, processing was important in order to best interpret the analog signal which is transmitted over a large range of values. With the newer digital displays, the processing is not as important. The signal is a 1 or a 0, so like with other digital links you either get the data or you don't. In fact, any processing actually reduces the display quality (like editing an image would). It may help the image look better in some situations, but will be worse in others.
The differences in price can be a number of factors. One would be features, like what input options it has and what kind of settings it has available. Another would be the components used, which will affect the longevity of the product. The exact method used to create the LCD will also affect both cost and quality.