If I'm calculating this correctly, S3 costs about $12.50 per 100GB per month (and that's not including any http requests you make). How is Dropbox (I assume they store data in S3) offering a lower price than that? I know they de-dupe data but is there enough common data being stored for them to be able to charge so little?
I pay for a 50gb account but use about 5% of it. I would imagine the vast majority of their users don't use anywhere near the full storage amount.
edit: not to mention shared folders. 45 GB of my account is a shared folder from work, which is probably shared among 10 users who all have to pay for the same data