I've got an iPhone 5 running iOS6 and I'm considering upgrading to 7.0.6. I see I need 3.9 GB to run the update and I'm pretty sure I'm going to get some of that back, but don't know how much.
How much space does an iOS 6.1.4 to iOS 7.0.6 take?
You'll get back all but about 60 MB of space. (Which is about the same space as all four movements of Shostakovich's Symphony No. 5 take up when copied to my iPhone.)
On my 32 GB iPhone 5, I now have 27.94 GB free space under iOS 7.0.6 instead of 28.00 GB that I had on iOS 6.