The 10" Octagon 30 Carbine was introduced in the Type 2 barrel era, 1967-1972. How popular it was and so how many they produced is anybodies guess.
Like with any TC barrel, supply and demand sets actual selling price. Find a collector that needs one to fill a hole in their collection, maybe a shooter that really wants one to play with bad enough and the price might be higher. Last 10" Octagon 30 Carbine I saw for sale about 3-4 years ago was "new in box" in the factory packaging (which ads value to a collector) and sold for under $200.
Who would probably have the most current information on availability and what they are selling for would be Ed Kirkpatrick (Ed's Contenders) or Ron Sable (RJ's Guns). Both great people to deal with.