I'm guessing this has been mentioned already somewhere (
EDIT: looks like
dave420 did just above mine...
) but I imagine that if they do release a 128GB iPhone, it'll be more than the standard $100 increase from the previous-most storage capacity, meaning they could likely maintain the profit margin that they currently have.
The move from 64GB to 128GB is an increase of, obviously, +64GB.
The difference in price between the 16GB iPhone 5 and the 64GB is $200 and the increase in storage capacity there is just +48GB.
So, while they could perhaps justify (at least to themselves) charging an extra, say, $250 for the jump from 64GB to 128GB, I think they could more easily justify to everyone else a price increase greater than $100 if they made that increase a flat $200.
Thus, I could see them pricing the next-gen iPhone as follows, assuming there's a 128GB version and the 16GB version is still offered:
16GB = $199
32GB = $299
64GB = $399
128GB = $599
Now, none of this is to say that I agree or disagree with such a scenario -- I personally don't have a problem with it -- but this is what I could see them doing.