[flurian]

It is cheaper to store 100GB in Dropbox than to store your own 100GB in S3. It would cost $12.50 plus transfer costs to use S3. It costs $9.99 and no transfer costs (besides your own net access) to use Dropbox. Dropbox is storing a lot more than 100GB, so they get bulk prices.

[mayneack]

They also take advantage of a ton of redundant data.

[diego]

Not for me, I only back up my original content on Arq (video and pictures, mostly).