Dropbox is running on top of S3. If you look at the S3 pricing per GB for storage and bandwidth, it's not exactly cheap. Back of the envelope, I'm guessing a 50GB account costs Dropbox something like $3-$4 per month. Their $9.99 plan can't really go down by an order of magnitude and still be profitable.
That, and there are a lot of the same files stored across accounts; DB stores it 'once' and delivers it to all the people with the same hash of that file.
S3 is really expensive. Dropbox really should get themselves their own infrastructure. There's no reason a huge company like Dropbox should stick with S3, and I'm pretty sure they're already planning/working on moving away from it.
