High end tonearms, perhaps even more so than almost any other hi fi item, are artisan crafted works of art. Yes, there is a science to the geometry of a tonearm, but once you have mastered that the specifics of a design come down to materials and the precision with which it is constructed. It's no surprise then that the particular manufacturer I favor (Joel Durand) is a professor of music and a musician himself. The selection of materials (woods and sapphire for example) and the precise way these materials are used in the design of a given tonearm are all critical -- even the size and location of tiny weights all make a difference that is easily audible once you have the system set up.
The economics of such a manufacture is the antithesis of mass production and economies of scale -- you are paying for a lone manufacturer producing in very small quantities. Are the prices justified? Are any prices for any high end component justified? Only if you believe that the improvements in sound you hear in your system are worth it -- personally they are for me (i.e. to be very clear I am arguing that high end tonearms are NOT a rip off in any way shape or form).
My experience has taken me from Linn->Rega->SME->TriPlanar->Durand and at every step I have appreciated the improvements along the way. Ability to achieve optimal alignment and optimization of every variable is a given at the high end (and frankly one of the shortcomings of the SME designs btw) but there is so much more to that in the art (word chosen deliberately) of tonearm design.
There's another can of worms to be opened about the specifics of the design (parallel vs pivoted, unipivot vs gimbaled etc etc) but my comments about materials choice and specifics of design apply within any particular family witness the extensive threads on the optimization of certain parallel designs for example.
