Currently Google's Music Beta is CURRENTLY FREE for 5gb / 20k songs. No pricing plan announced.
Currently Amazon's Cloud Player is FREE for 5gb / 20k songs.
$20/20gb... $50/50gb.. etc. (Basically $1 per GB).
Right. Apple also gives you 5GB free, though because they actually made deals with the record labels, you can't store ripped music in it. $25 at Apple gives you unlimited storage. Google has announced that they will charge for the extra space, they just haven't announced how much, and Amazon's is, obviously, more expensive once you go over 25GB. And you don't have to upload most of your music with Apple: higher quality versions are already there.
The real trick will be this: the record labels will sue the hell out of Google and Amazon, and their programs will radically change.
On the 5GB that's free from Apple, in addition to the things
falcarrion listed, it will also be used by 3rd party apps to perfectly sync their state. As examples:
You're playing Angry Birds on your iPhone and you pause as you get off the bus. You walk into your home and sit down at your laptop or desktop computer and when you launch Angry Birds it's paused at the exact same place, with the same score and status and all, where you continue playing. After a few levels you pause it again and later that night while watching a boring tv show you pick up your iPad and launch Angry Birds again where it's paused at the same place you left it at earlier.
Or you're editing a word processing document on your laptop; you've made a variety of changes and have some text selected when you have to leave. The next day on a desktop computer at work you open the file from the could and not only is it scrolled to the same place and the same text is highlighted, but the Undo command steps you back through changes you made on the laptop the day before. Later that night you open it on your iPad and poof, same thing.
It's a lot more than a place to store some music, or even store some files.