199$ - 16GB
299$ - 64GB
399$ - 128GB
would make a lot of sense. 100$ for the first additional 48GB, 100$ for the next 64GB.
(Right new it's 100$ for the first additional 16GB, 100$ for the next 32GB, which is a bit uneven, as well as expensive.)
The steps you see are OK, but it's so funny you Americans always forget that these prices are not the real prices. They are:
$649
$749
$849
You pay the difference via the monthly Carrier fee!
No wonder you always buy wooden Houses you can't pay... ;-)