A 50mm f/1.8 prime from one of the major manufacturers can easily be found for $200 or less, but zooms that include 50mm in their range always seem to have much smaller apertures even at 50mm until you get into the thousands of dollars, and even then you're probably going to end up at f/2.8 at best. Is there a technical reason why a cheap 18-55mm kit lens can't be f/1.8 at 50mm and go down at the wide end?
My understanding is that 50mm is the simplest (and cheapest) focal length to design a fast lens for, at least based on prime lens prices; fast lenses that are any wider or more telephoto quickly become more expensive. Nikon's cheap 18-55mm zoom lens is f/3.5-5.6, but how difficult would it be to build an f/3.5-1.8 instead? That is, a lens that is f/3.5 at 18mm and f/1.8 at 50mm. I don't think I've ever seen a lens with a maximum aperture that grows when you increase the focal length, but it seems potentially reasonable when you start really wide (where a large aperture is presumably difficult to accomplish) and end around 50. Do any "reversed" non-constant aperture lenses exist? Is there something about the optical design that prevents this?