Headlight covers are made from a type of plastic called polycarbonate which has great impact resistance and transmits light almost as well as glass. However it scratches easier than glass and, like most plastics, deteriorates when exposed to ultraviolet sunlight.
Polycarbonate headlights are given a UV-resistant coating for protection which can last for years. Over time, grit, dirt, harmful chemicals thrown up from the road, frequent washing and especially UV light exposure gradually wear the protective coating away. Once the polycarbonate is exposed, it starts to acquire that cloudy appearance.
(Prices may increase due to size and/or condition of vehicle.)