While some of it is marketing, a lot of the price difference is in the amount of glass that has to be ground.
Remember, a 2 inch increase in aperture is not just 2" in aperture, it is a big jump in surface area and if you pay attention, as the dobs get bigger, the focal ratio gets faster so not only is there a much bigger area which requires more glass to be ground, but the faster speed means that the curves have to be deeper, and well, that means even more glass that has to be ground and polished, and the raw amount of material that has to be ground away gets pretty huge.
(Figures.. An 8" f/6 has about 50 square inches of surface area, where a 10" has about 78 square inches of surface area, so it really isn't just 2 inches... It is a 156% increase in surface area an that surface has to be ground deeper in the 10" than in the 8" because the curves are f/5 vs f/6. That does not sound like much, but it is a lot of extra grinding.)
So, you go from 8" with an f/6 curve to a 10" with a steeper f/5 curve, then to 12" with an f/4.9 curve, then to 14" with an f/4.5 curve and well, as you can see, you have to grind far more glass away. Again it is area and the depth of the curves that make bigger mirrors so much more expensive.
See, the machine can make one mirror at a time. Now if I make one big mirror and I have to grind 10 times as much material away, that means I have to have that machine tied up for far longer, and that means I cannot make as many mirrors, and I am still paying the same amount for my manufacturing facility, my workers, my marketing people, my insurance, my sales people... Get it? So, If it takes a lot longer to make one scope, I have to get as much profit out of it as I would get out of four smaller scopes.
The sheet metal in one of these dobs costs few bucks, and the entire production cost of the truss tube is probably $50. It is the mirror where the production cost is, and a big mirror takes a whole lot of grinding.
Area to be ground and the steepness of the curves.
Edited by Eddgie, 19 January 2020 - 08:36 AM.