From what I can gather, though I have never used Elixir strings myself, the Elixir strings are guarded against contaminates from getting between the gaps of the string winding. They use a special kind of heat shrink around the entire string, rather than just a coating on the winding itself. I'd say this is the cause of the cost.

http://www.elixirstrings.com/whyelixir/czot.html#