I would say that Dropbox (being a really super company with a great product) has issues that are actually even larger than the storage cost.

Dropbox is based on Amazon S3, which means that not only do they have storage costs to a 3rd party that are so to speak 'out of their control' but they are also dealing with bandwidth and transaction costs.

I wish them all the best, however I can imagine this being quite expensive for them considering the amount of free users they have.