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

Lewm, The brilliance of the LT is that it is using the energy from groove friction to drive the arm! Friction pulls the horizontal bearing platform forward and a magnet underneath makes sure it maintains the right position. 
Uberwaltz, I am not entirely sure but I think the alto is a version of the CB.
The CB is as good as a pivotal arm can get for several important reasons. 1st is the vertical bearing is at the level of the record which limits warp wow. 2nd is it is a neutral balance arm. You can draw a line from the center of the counterbalance through the vertical bearing right through the center of the cartridge. Most other arms are static balance arms which always try to fine one level whereas a neutral balance arm will stay where you put it and won't start bouncing. 3rd. It has a brilliant magnetic anti skating device. No friction. 4th it is not a unipivot and has excellent bearings. Need I say more?

And Uberwaltz, Franks other arm are also genius. Check out the CB. You can get one for $5000 and it could easily run for best pivotal tonearm made. 

I think they are somewhat less. I have a relationship abroad who say I can get one for $10,000 (wink wink). I'll have to pay VAT tax up front but I'll get it back in several months.
The dealer in the States is Xact Audio  https://www.xactaudio.com/analog/
He sells it with or without his own turntable but he modifies the arm replacing the wood for a magnesium arm tube which I am really not crazy about.  He has some kind of relationship with Schroder apparently he can sell them here in Magnesium and Frank can sell them in Europe with wood. As for his turntable there is too much mystery about it for me to be sure. I think if I could afford to today I would get a Dohmann Helix 2 and put an LT on it. That would certainly be a last stop turntable.
Not to worry Uberwaltz. This type of design will work it's way down market. Reed is working on a less expensive version now called the 5A.
It is another passive design energized by groove friction but it uses a dizzying series of arms and bearings to do the job. The Schroder is certainly a more elegant design. You have to love the magnetic track trick. One magnet + one bearing vs a bunch of arms and bearings.
The 5T is however a beautiful arm and boy does it work. It is $20K and very complicated but still a grand exercise of applied technology. 

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 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, 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.  

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!  

Lewm it is just the opposite. A shorter arm setup is more critical as the offset is higher leading to higher skating forces and an error in overhang will lead to greater tracking angle errors. 

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.  

Right