Looking at prices on newegg and amazon, there are several 27" monitors to choose from in the $300 range. If you look for 30" monitors, the prices jump up to $1000 or more. Is the price increase due to lack of monitors at 30" or is there something else at play?
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Part of the reason the cost increases is because of the cost of the materials. Even if the number of pixels remains fixed, there is more surface area to manufacture, test, and ensure remains illuminated.
Moving from 14" to 17", for example, is a smaller increase than moving from from 27" to 30". Even though the diagonal change is still 3", the surface area is not equivalent.
Assuming a 16:10 ratio, a 14" screen will have the dimensions of about 11.8" x 7.4". Moving up to 17", the dimensions will be about 14.42" x 9.01". That's a difference of 42 square inches.
For the 27" screen, the dimensions are 22.90" x 14.31". The 30" screen's dimensions will be about 25.44" x 15.90". That's a difference of about 77 square inches.
So the cost difference will continue to increase, at least according to the increasing amount of materials needed for each additional inch in the diagonal measurement.
Resource supply, increased complexity of the electronics and controllers, and resolution will also play a role. It's complex, but at least with larger and larger panels, it really does take a lot more investment for each additional inch than it used to.
The main cause of the price jump in the cost it the cost of the panels. The larger a panel gets the cost goes up dramatically, similar to die yields of silicon chips. Economies of scale also play a large factor, as there is a lesser demand for large panels less are produced driving up costs, etc.