There are two main difference's between cheap halogen lights and the more expensive ones. The first is the quality and placement accurracy of the material used for the reflector coating. The second is the grade of materials used for the lens.

High quality lights usually have better quality material for the reflector coating that is electrostatically - vacuum deposited to give about a 10 to 100 micron surface variation on the reflector.

The more important difference is the material the lens is made from. lenses are not just glass, although some of the cheap lights are. The best quality lights use a lead crystal lens with very high refractive indices (RI). With a very high RI and an accurately molded lens, this allows the light beam to be shaped very accurately. This means you can have your lights adjusted so oncoming drivers are not dazzled, while you have excellent night vision even on low beam.

I designed some lighting systems for overnight haulage trucks for one of the major freight compamies a few years ago and had to do lots of home work to get the system to meet ADR's (Australian Design Rules).