Every iCloud user is given 5GB of free storage, and that can be upgraded to 20GB of storage for $40.00 a year or to 50GB of storage for $100.00 a year. I currently have my account upgraded to the 20GB of storage, as I was pushing the limit of the free 5GB of storage that was given to me.
Recently someone I know made the switch from the Microsoft world to a Mac. The transition slowly starting with an iPad, then an iPod touch, and then eventually he bought a Mac for home use. A conversation about iCloud came up between the two of us, and he was complaining about the amount of storage that Apple offers for free. He voiced his concern that he has these multiple devices and only 5GB of free iCloud space. His thought was that if he buys multiple devices then he should get an extra 5GB of storage on iCloud for no extra charge. My response was, you only get 5GB and that is just the way it is. If Apple gave away 5GB for every device then almost no one would pay to upgrade their iCloud, and Apple wouldn't make money off of the service. (I assume that Apple doesn't really make money off of iCloud as much as it probably turns whatever profit they receive from iCloud upgrades into maintaining the infrastructure.)
Since having the the discussion I have started giving it some thought and I have started to change my feelings towards the subject. I still feel that 5GB per device is an excessive amount for Apple to give away for free, and that it would never happen. However I do think that Apple could compensate multi device users with additional storage space to a point.
In my case I have two mobile devices, and two computers. OS X does not fully utilize iCloud for storage, but I would guess at WWDC this year it will be revealed that the Mac is getting more iCloud integration and therefore it will take up more of that storage footprint. I don't really now what a reasonable amount of storage would be for Apple to give out, but even another 5GB for free would be welcomed. The problem however with this whole philosophy is how does Apple determine how much space to give someone, and what happens if that person sells their device, should they be able to keep their upgraded storage since they have one less device?
If I look at my iCloud usage the only OS X apps that are using iCloud storage are Pages, Keynote, and Pixelmator. I had recently considered purchasing Together 3 which I would chosen to use iCloud to keep all of my stuff in sync between all of my devices. Something like that would use a significant amount of storage over time. If I go to my iCloud settings and look at my total storage used is, it is 8 GB, so I don't even need the full 25 GB that I currently have, but I need more than the 5GB that was given to me.
Since developers have such a hard time trying to implement Core Data syncing I don't think that this is such an issue right now. This is because a lot of developers choose to provide access to Dropbox, or they build their own syncing services for their apps. The majority of an iCloud users storage is taken up by iOS backups, but if Apple finally gets Core Data right and developers start using iCloud more for their app data, then I think people are are going to start finding the need to require more iCloud storage for their devices.
Whenever Apple decides to make iCloud functional for developers to use the service to sync their app data I think that they are going to have to either give storage increases for people who buy multiple devices or offer a wider selection of upgrades for storage, such as more tiers than their current offerings.