Although I found the price decreasing by 90% at each change somewhat distracting. 2TB of data costs only 10% more to store than 1TB? People with less than a terabyte are subsidising the big boys.
It's more like storage isn't the driving factor of the costs. At low end, you'll have some fixed per-user costs; as you go from mid to high end, you may enjoy some economies of scale yourself, and you may want to pass some of those savings on to your customers.