My wife and I currently have iPhone 5 phones (she has a 16 GB and I have a 64 GB). The primary usage of space is the photos/videos that are shot with the phones and really just the discipline to download them to our server computer every now and then to conserve space. I've had mine go up to 43 GB total space used, but when all offloaded, I get down to about 10 GB.
How much music I load up also can be variable. A few playlists can keep me at under 1 GB for music, but if I had the additional space, I can easily add 12-15 GB. Not really needed though. And while it streams TONS of data, the iTunes Match is an option too ($25/year to conserve space).
Regarding iCloud, from what I read, it only backs up photos, but not videos...is that true?
I think you can set DropBox and Google Drive to upload any file type, but I need to confirm and look into it.
Here's some pricing links:
iTunes Match
http://www.apple.com/itunes/itunes-match/
$24.99/yr
iCloud
http://support.apple.com/kb/HT4874
5 GB free to start
10 additional GB (15 GB total): $20/year
20 additional GB (25 GB total): $40/year
50 additional GB (55 GB total): $100/year
Dropbox
https://www.dropbox.com/upgrade
2 GB free to start (can get referral space too)
100 GB for $99/yr
Google Drive
https://www.google.com/settings/storage/summary?hl=en_US
5 GB free to start (can up to 15 GB with Quickoffice)
100 GB for $59.88/yr