your question is something I never thought of...still lets see if i can answer 1st one
large aperture lenses are costly due to larger lens element
The size of the aperture increases with the focal length....a 55-200mm f4-5.6 have nearly 1.5 times the size of front lens element then a 50mm 1.8...but a 55mm 2.8 lens element is very small compared to 50mm 1.8
The size of front element in a zoom lens will me the max aperture possible...in case on 55-200 f4-5.6 the front element will be of 200mm f5.6 size
A prime lens are very simple in construction soo even with bigger apertures they are cheaper...but a zoom lens are complicated with much more glass inside...if u use fixed aperture in zoom all the elements need to be of bigger aperture size...which becomes costly to make.
this is the same reason max aperture in a zoom lens is f2.8 and in bigger telephoto lens its f4 ....just check the size of nikon 400mm 2.8 then think how will be the zoom look if its 100-400mm f2.8
*photographylife.com/wp-content/uploads/2011/10/Nikon-Telephoto-Lens-Comparison.jpg
here you can see from left - 70-200 f2.8 , 300mm f4, 300mm f2.8, 200-400 f4, 400mmf2.8 , 500mm f4
just see the size difference between 300mm f4 and 300m f2.8
about flicker in tv and mobile...I dont have much idea...I think its due to some refresh rate frequency