The same "rate table" shown at "http://www.alentus.com/backup" is used to determine the costs for increasing your quota incrementally (for example, adding 10 GB to your 10 GB Plan) as is used to determine what the costs are for the various SecureStor plan levels (for example: choosing the 25 GB SecureStor plan).
It is always cheaper to choose the next plan "level" as indicated on the "rate table" than it is to incrementally upgrade to that same level. For example, to incrementally go from 10 GB to 25 GB quota, that would mean two 10 GB plans at $54.95/mo and one 5 GB plan at $29.95/mo. for a total of $139.85/mo. Simply changing to the 25 GB plan would cost only $99.95/mo, a $39.90/mo. savings.
Note: Rates are subject to change.