A good portion of that expense can be understood by the two words in the upper-left corner of the protector. It says "Schott Glass".
Schott AG makes some of the finest optical glass in the world. Zeiss uses it for their lenses. Back when I worked for a company that made projection systems for flight simulators, we used a Schott Glass element in the optical path between the CRT and lens. If you want/need the best, go with Schott.
Yeah, it's expensive, but you get what you pay for.