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

I never realized the 12" version is more expensive dis-proportionally. Miller, you're very correct about bi-amping! Similar to when stereo came out and a husband had to explain to his wife why "they" needed another amplifier and another Hartsfield in the living room!!

You can buy a "12 inch or "10.5 inch tonearm if you will ignore overpriced new items and will look for a used items that sells for less.   

And you can always buy classic Japanese vintage high-end tonearms for much less without any loss in quality (quite the opposite sometimes). 

There are many long tonearms that does not cost too much. 
In many cases the only difference between long and short is one part - the arm tube (which is longer and slightly different geometry), the rest is the same. 

There is no logic explanation why the same tonearm with longer arm tube sells for triple price (for example FR-64s and longer FR-66s) and not only $200 more as it should be. 

If they can sell you same tonearm with longer arm tube for double price then why not to sell? It's marketing. 

Small manufacturers like Thomas Schick in Germany will not charge you too much for his brand new "12 inch tonearm (it was about 1200 EUR if i remember correct). 

If you think the only good tonearm is the most expensive one then i can't help you.   

Because they can.

And with vintage SME tonearms the price differences are ridiculous between the 3009 and 3012 arms.