They bumped the storage last year. Not happening again this year.
They just added a 64 GB, they didn't lower prices on the current capacities. That's what the OP is asking.
Here are some graphs of flash memory prices, from a bit before the 4S release till now:
Is that enough of a drop to compel Apple to move to 32/64 GB? I have no idea, but it is interesting that the 64 has essentially halved in price from last summer. A $299 64 GB would be insanely great, but I don't think it'll happen quite yet.
Of course, if Apple really wanted to, they could increase the capacities irrespective of the price drops--they already have insane profit margins.
Considering the cost of memory has come down, and the fact that with each subsequent iPhone release photos and videos take up significantly more space, we amass more and more apps and music, and there is no removable storage, it will be pretty disappointing if the base storage isn't bumped up to 32GB.
Not that I'd be surprised if they don't, this is apple after all. But it's the fair thing to do. If the galaxy s3 can have removable sd cards, apple can at least stop gouging us on memory and transition to a 32/64 option instead of 16/32.
Question is, what is their cost relative to 64GB this time last year?
It'll only happen if the price of a 128GB chip is the same price as a 64GB chip was last year...etc...etc.
I was always about the lowest amount of storage (for price purposes) until I purchased the 4S 32GB (because 16 was out of stock and I wanted it THEN) with the improvement of the pic/video I take so many more - which has caused me to fill up the 32GB space - I really hope the 5 starts at 32.
Like others have said - 16 seems most likely.