It's worse than that, though. If the manufacturer's cost to produce a consumer item goes up by one cent, the final retail price of that item will generally increase by something between ten and twenty cents -- because the original penny gets marked up by anywhere from 3 to 10 middlemen, each of whom price in their "costs" for changing the price as well as the actual increase in cost to them. This is why, for instance, a plastic camera that's cheap enough to give away with a roll of film still sells for $5 to $25 if you buy it separately.
Otherwise, however, the economy of scale is one of the major factors in why falling demand results in higher prices -- because production must be reduced along with demand, or a manufacturer gets stuck with a bunch of product that can't be sold immediately and, in the case of film and such, will be impossible to sell after a certain time in storage (and meanwhile, those fixed costs are still running).
When there's only film manufacturer left on Earth, film prices will be similar to those of glass plates now -- last I heard, the ones from Russia are $8/plate, sold in boxes of twelve. Sheet film could reach that price at some point -- but I won't be a photographer at that point, unless I'm making my own materials.