The way memory is made, to double the size is practical, but, to make say 64 then 96 would use a different process entirely. So that's why they double the size. As far as $100 dollar allotments to each doubling...not sure that matches anything except consumer marketing trends.
As far as ppl saying you don't need more than 16GB, I say, no way, I'm maxed out, always having to take stuff off. Between under 1000 songs at the reduced 128kbps rate, apps, photos, videos, it's nearly full all the time. I wish I would have spent the extra to get 32GB.
Plus, they keep improving camera, photos & videos keep taking up more space, so, my opinion:
5C 16GB, 5 16GB, 5S 32/64/128GB ...but, the 5C could come in at 8GB...I just don't think Apple will up the high end and not drop the low end...why by five types of memory when you can just do 4? (or, if you look at iPhone & iPad, maybe they will add 256GB to iPad and drop 8GB on iphone and 16GB on iPad?).