Why Do 12" Tonearms Cost So Much More Than 9"?

Lewm it is just the opposite. A shorter arm setup is more critical as the offset is higher leading to higher skating forces and an error in overhang will lead to greater tracking angle errors. 

12 inch arms have less offset, so skating settings less critical--or use none at all (VPI).  They produces less distortion throughout the recording, though the difference is small, than shorter arms.  Once set for optimum SRA they change less than shorter arms for changing record thickness.  

On my 12" arm I use the same mirrored protractor I used on a shorter arm.  Modern version costs "the small fortune" of $20.

Your money; your choice.

Mijo, Your objection to the rumor I repeated is noted, but I need to see and/or do the math before you can convince me.  Some pretty obsessive tonearm alignment persons on the internet have claimed the opposite of what you say.  My guess is that it depends upon what kind of mounting error is made, as to the effect of that error on a short tonearm vs a long tonearm, but like I said, I haven't done the math.  And I don't want to do the math.  I am kind of an alignment nihilist in the first place.  I really don't give a hoot.

Forgot to say to Elliot, that Blackbird tonearm is "controversial" to say the least.  You might have asked here about it before taking the leap.  I think Chakster has some info.  I hope you'll be happy with it.

mijostyn

Schroder LT.
The Reed 5T is another example. These arms stay tangent to the record, do not skate and totally avoid the problem of very high horizontal mass which plagues the vast majority of designs.

Any pivoted arm is subject to skate, a force that is the result of both the pickup arm's pivot and its offset, if any. None of this is a mystery and an arm's skating force can be measured.

There have been a variety of "pantograph" type arms over the years, the infamous Garrard Zero100 being one especially unpleasant example. I don't have any experience with any of the newer tangential tracking efforts, which include the Nasotec Swing headshell and Klaudio arm. But in the past, a prime problem with these types of devices has been friction, so the cure was worth than the disease.