First off, the cost: FREE (as in beer).
What is Cloud Drive? Think of it as a flash drive that you don’t have to carry with you; one that is instead stored on the internet, so that you can easily peruse the files wherever you are (assuming you have internet access).
If that alone isn’t enough (and for many people it may not be), the real reason you want to sign up is that AFTER you have signed up, any new purchases through Amazon MP3 are also stored there for free (as in they don’t count against your cloud storage limits). In essence this means that when you buy new mp3s from Amazon, they are stored in the cloud in perpetuity, so you can re-download whenever you want (if for instance your computer crashed and you lost everything that wasn’t backed up, or if you wanted to download it from a different computer, etc, etc).
In order to take advantage of this, you need to do something slightly different when making your first purchase (select Save to Amazon Cloud Drive rather than Save to Device) or you can follow the steps below.