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

Cleeds, The Reed 5T and the Schroder LT do not have fixed horizontal bearings. Both arms are free to move horizontally just like any pivoted tonearm. I'm not sure how the Reed is set up but I do know all about the Schroder which I am itching to buy. It comes with all the tools necessary for set up including a special very flat blank record. Once you have the turntable level and the tonearm set on tangent you play the blank record. There is an adjustment to the pivot platform that slightly changes the radius of the Thales circle. You adjust it until the arm does not skate at all. This does not account for the irregularities that most records have, warps and such. So, there will still be slight skating forces one way or the other but this is true of any straight line tracker. The main advantage of these arms besides virtually no skating as we normally think of it, is the stylus does not have to drag a very heavy carriage along.
The Schroder design is an example of brilliant lateral thinking. Check out the patent I linked to above.  

mijostyn

... The Reed 5T and the Schroder LT do not have fixed horizontal bearings. Both arms are free to move horizontally just like any pivoted tonearm.

That’s pretty obvious. But if the horizontal pivot itself is fixed, then the arm will generate some skating force. That’s explained in the link I provided you. Skating force is the product of both offset and the pivot itself, which is something you seem to overlook.

Skating force is dynamic. That the Shroder arm can be set so that it has zero skating force at one point in its arc of rotation does not mean it has zero skating force across the entire arc, especially because skating force is affected by other forces such as VTF, which will also vary across the record. Again, this force can be measured, so its silly to dispute it. And more information is in the link I provided.

Cleeds, I just finished reading your link. It is a very lucid explanation of how skating force is generated. It also mentions both Reed and Schroder.
I miss understood what you meant by fixed horizontal bearing. Both the Reed 5T and Schroder LT do not have fixed horizontal bearings. The platforms that the horizontal bearings are mounted too are hinged at the right radius so that the pivot point of the bearing follows an arc as dictated by Thales thus the arms remain tangent to the groove at all points. This is shown by your author.
Your author relates the skating force should be 10 to 12% of VTF. 
Wally Tools says 9-11%. Some say the Wally Skater is the most accurate way of adjusting skating  https://www.wallyanalog.com/wallyskater
I have not tried one. It costs $260.00. It does however reliably gauge anti skate as a percentage of VTF. I use the Hi Fi News Analog Test Record and use the lowest velocity band to set anti skating. It coincides with Frank Schroder's method of using the run out groove area which Peter Ledermann also endorses. 
At any rate anti skating is a ball park measurement. There is no exact right figure and many opinions as which side of the ballpark you want to be in. I'm happy just to get in the ball park. 

Cleeds, before you suggest that I am silly I suggest you carefully review Schroder's patent so you know what you are talking about.

Cleeds, I am not sure what you are thinking, but just to be as anal as possible, I need to say that pivoting per se is not the cause of the skating force.  The skating force is first of all due to the friction between stylus and groove; I am sure we agree on that.  The friction is in a direction away from the stylus tip; the friction force wants to pull the stylus and cantilever away from the cartridge body and tonearm.  To resist that force, a counter-force is exerted by the cartridge/tonearm in an equal magnitude and opposite direction. The skating force is a vector generated as a result of that force resisting friction, because of the fixed headshell offset angle and also because of lack of tangency of the stylus to the groove. When a straight line can be drawn through the longitudinal axis of the headshell to the pivot point, and when the cartridge in that headshell is tangent to the groove walls, there is no skating force.  Among "conventional" pivoted tonearms, only underhung types, which all have zero headshell offset, ever meet that criterion in actual practice.  For such tonearms, that condition applies for the brief moment when the stylus is tangent to the groove.  For pivoted tonearms that work like the Schroeder LT or apparently the Reed 5T, that condition is constantly met across the surface of the LP, if they are perfectly set up. So, ideally (given diligence in set-up), those latter two tonearms would not generate a skating force.