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

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

The most TT's are made for one single 9'' tonearm. Some like
SP 10 needs 10" arms. But for those who own an TT with
single 9'' or 10'' arm but want a second arm there is no other
option than 12'' arm on an armpod which can be only installed
next to the TT. 

Right

Woops, Brain fart!

Hi guys. I bring greetings from French Guyana.

I'll side with Lewm on this one. Because the people who will go for a 12" arm will pay those prices. It is all about what the market will bare. I'm going to start a new thread on cartridges shortly which brings this into focus.

Is a 12 inch arm better? Overall with medium to low compliance cartridges, a little but it depends on the arm. In many cases not at all and in some, worse. The real jump is to arms that can stay tangent to the groove and are straight avoiding the skating problem altogether. There are several arms available that utilize Thales geometry and normally pivoted but straight arms my personal favorite being the 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.  

Since SME was mentioned, when new the price difference 
between the more commonly sold 3009 and 3012
was not a multiple of its additional length.
 
As years passed the greed factor followed exponentially so
lets be honest.