If you are considering a cheap way to store documents online, considering using Google Docs, which now allows you upload documents to store online. Prior to the change, only documents created with Google Docs could be stored online.
For solo and small firm practitioners, the service is a great way to allow multiple users to access files. Unlike Dropbox, Google Docs only stores the documents in the cloud. If your internet service is down, or you do not have internet access, you cannot access files stored with Google Docs. That can be a potentially crippling problem to a law firm.
That said, uploading to Google Docs is easier than using a thumb drive and arguably more secure (depending on your view of cloud computing security). Rather than emailing documents back and forth, you can store collaborative documents online to make working together a breeze. Dropbox is superior, but if you are considering storing documents online, try Google Docs for a free foray into cloud storage.