I am sure that Dropbox is able to squeeze some extra efficiency out of their storage requirements by storing only one copy of very frequently stored files, but it is a long way from there to "printing money legally."
Plus, I doubt that anybody on Dropbox is maxing out their storage, much less everybody; any analysis based on that assumption is doomed to failure.
I agree, I highly doubt that the percentage of storage that is non-unique is enough to increase Dropbox's profit margins significantly.
I'm certain they use this and several other techniques such that each user reflects a far less impact on Dropbox's storage than the 50GB bought, but if that's printing money than Amazon would be a first world country by now.
I am sure that Dropbox is able to squeeze some extra efficiency out of their storage requirements by storing only one copy of very frequently stored files, but it is a long way from there to "printing money legally."
Plus, I doubt that anybody on Dropbox is maxing out their storage, much less everybody; any analysis based on that assumption is doomed to failure.