Geek, I know that the latter two models have an extra 4gb of RAM each (8gb vs 4gb on the smaller models). Might be other changes too, not sure... But, it is a reality of current SSD's that the price increases at a faster rate than their size.. You can get four 128gb SSD's for quite a bit less than a 500'ish gb SSD.
Any which way, all of these storage options are overpriced, but that's par for the course. Some companies offer size upgrades for $50'ish, but most stick to $100 per size increment and rake in the cash off of users in doing so.