The reason Airstream Travel Trailers are so expensive and usually much more expensive than other travel trailers are is because of the materials they're made from and the quality that the Airstream Travel trailers are made.
The materials that Airstream travel trailers are made from such as aluminum which is riveted on the inside is more expensive than say fiberglass and wood that the standard travel trailers are made from.
Airstream travel trailers are also more labor intensive to manufacture than other standard travel trailers and requires more work and good quality builders who are trained to build the Airstream travel trailers.
Airstream Travel Trailers are worth the money you spend on them because they will typically last a lot longer than other cheaper models of travel trailers and they hold their value a bit more than a standard travel trailer does.