As an MU site owner you bear the costs of the servers and storage space that your bloggers/users are using to upload their content.
When you use Cincopa plugin each of your users will get 400MB of free space each hosted on our servers. When one of your users upgrades his account we will give you your commission. So not only that you don't pay for storage, you even get money while providing your users a professional tool and CDN.
You can track your users and affiliate commission here http://www.cincopa.com/cincopaManager/affiliates.aspx