Well I can divulge how cheap I've managed to get my upgrade now it's all done and dusted. Before I didn't have the figures.
I did the following:
10 days before warranty went have phone replaced by apple.
Sold phone as mint, boxed etc
Purchased iPhone 5 off of someone I know off of here
This leaves me with 4 months contract left on Voda (I was on 18 monther iirc)
In 4 time months I can almost half my plan to £26 on a rolling 30 day contract.
My Method:
Total cost to upgrade (taking into account phone sale): £150
Line rental over 12 months: £372
Total: £522
Equivalent cost to do this on Vodafone by upgrading and paying off contract:
Pay off contract: £164
Purchase handset (taking into account phone sale): £-11
Line rental over 12 months: £492
Total: £645
Fair saving there, and I'm free to mess around with my contract however I want