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

It probably depends a lot on the design of the arm, and the available tooling etc, whether a longer arm tube is very difficult for the manufacturer to make. Clearaudio Universal is an example where the price difference is reasonable - the 12” version is less than 10% more in msrp compared to the 9”. But the 9” is more versatile as it can also fit on the fixed arm board models like Ovation and Performance DC.

Graham Phanom is another arm with a reasonable/small price increase on the 12” version. 

The Triplanar price increase sounds sadistic.

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.