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

Show me a pivoted tonearm that has a fixed zero headshell offset at all points across the surface of an LP, and I will usually show you an underhung tonearm.  Underhung tonearms do generate a skating force, along with lots of tracking angle error, even though I like the ones I've heard a lot.
The pivoted tonearms that incorporate mechanisms of one kind or another to maintain tangency to the groove mostly do so by changing the headshell offset angle as the tonearm moves across the LP.  The headshell offset angle, so long as it is not zero, per se generates a skating force at all times.  That was my point. Now you mentioned it, I looked for photos of the Schroeder LT, which I know works by changing the pivot point rather than headshell offset, and indeed it does have a fixed zero headshell offset angle.  So therefore it would generate no skating force.  Maybe the Reed works similarly; I haven't googled a photo.  There was some momentary fuss about the Schroeder LT when it first appeared, but you don't hear much about it these days, maybe because Fremer didn't fall in love with it.  Pretty cool idea.

So why did you disagree with Mijostyn in the first place?  Mijo did say that pivoted tonearms that maintain tangency to the groove generate no skating force, which we now can see is partly correct, for some examples but not others.

As regards your intimation that a pivoted tonearm per se generates a skating force, that is not really correct, if you consider that an underhung tonearm, at the one point on the surface of the LP where the stylus is tangent to the groove, generates no skating force, because no headshell offset angle.  It is also not correct for the class of overhung pivoted tonearms that maintain tangency to the groove by virtue of altering the position of the pivot, a la the Schroeder LT.  All other overhung pivoted tonearms do generate a skating force by virtue of their overhanging the spindle, combined with their having a headshell offset angle that is not zero.

What "measured error" are you talking about?

Cleeds.
I gave you an example in my post.
I thought it was obvious my statements were just related to maths and I was not discussing any actual tonearms or tracking errors as related to turntables per se.

Lewm.

I say that it does not seem to be borne out in practice just from my own experience with 9", 10" and 12" tonearms.
I cannot prove it one way or the other and similar to yourself I truly do not care overmuch.....

Cleeds.
Just to clarify my thoughts maybe a little better and to follow where I believe Lewm is heading.

Maths would seem to indicate that any INITIAL error in setup on a longer arm would result in a greater playback error.
So a basic change in vta by eye just looking at, say making the tonearm more tail down would result in more change at the stylus the longer the arm is.

But as I have said from practice this just does not seem to occur.

And this maybe because I am missing something ?

The Reed 5T and the Schroder LT are very similar arms. They work on the same principle which you can review here  https://www.youtube.com/watch?v=oIoHvCE9F0I
The Reed is animated while the Schroder is entirely passive. Both arms are pivoted. They do not suffer from horrendous horizontal effective mass. Both arms are dead straight and if set up correctly do not generate any skating force. They have to be right on the tangent, the turntable has to be dead level and the record has to be dead flat. All of these parameters are unlikely to be met so there will always be a little skating force one way or the other but magnitudes lower than any offset arm. 
The Schroder arm is brilliant. It uses groove friction to operate it's horizontal pivot platform and a magnet to maintain the correct position. It is very hard to describe but you can see the patent here   https://patents.google.com/patent/US8576687B1/en.
Lewm, both tonearms maintain tangency within 0.05 minutes which is pretty incredible. Both will easily out perform any offset arm not to mention any straight line tracker of any type because cartridges do not have to suffer under the high horizontal effective masses these arms have and are subject to very little skating force.
I am leaning towards the Schroder LT. It is 1/2 the price and just an elegant design. The Reed has one major advantage. It has removable arm wands like Graham arms and you can get them in a range of effective masses so you can tailor it to any cartridge. Switching from one cartridge to another is a breeze. Obviously the Schroder is going to be more durable in the long run. 
Uberwaltz, you are not missing anything. 12" arms are less critical of set up. We all agree they have less tracking error I hope. This means that if a cartridge is misaligned by 1 degree this will have less of an effect on a 12 inch arm relative to a 9 inch arm. Frankly, you either get the cartridge exactly where you want it or let someone else do the job who can. There is no magic here. Anyone with two functional hands and two functional eyes can do it. Just make sure the stylus guard is in place and save the beer for after the job is done unless you have an essential tremor than drink the beer first!