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

Uberwaltz, The analogy you gave for why measuring errors are magnified in proportion to distance is exactly what I have in mind as a reason why errors made in mounting 12-inch tonearms might be more consequential than errors made in mounting 9-inch tonearms (and also mounting and aligning cartridges on either type).  But why do you conclude that this proposition "does not seem to be borne out in practice with tonearms"?

uberwaltz

Simple math states that from a pivot point of A to a fixed point of B that the greater the length between A and B is then the greater the measured error will be at point B from a change at point A.

What "measured error" are you talking about?
If we’re talking about tracking error - deviation from tangency between stylus/cantilever and the record groove - the longer the arm, the less the error. That’s because the longer the arm, the greater the arc its pivot describes; the bigger the arc, the less the tracking error. It is simple geometry and the reason d’entre for a longer arm.
lewm

... the skating force would result from the constant fact that there is a head shell offset angle which is changing all throughout play.

Some of these pivoted arms have no offset. There’s still some skating force, by simple virtue it being a pivoted arm.

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.....