On that basis the iPhone 5 comes out a little cheaper on contract with Three over 2 years (£37/month + £90 upfront for 32GB) plus the free interest on the best deals although you can pay a couple of hundred more over the time with other providers.
For the iPhone 5 the 2 year cost is about £1000 including service for a full offer but only a few limited SIM only deals are much cheaper. Your deal would still be £840 for data only and a 32GB phone even if the prices don't go up.
In the UK you should check out http://giffgaff.com/ - £10 a month for unlimited internet, unlimited SMS and 250 minutes of calls (mobile or landland, and free to other GiffGaff numbers) - not a bad deal at all. Runs on the o2 network, so pretty reliable.
£120 for 2 years is an amazing deal, and if you can buy your iphone 5 (or other mobile) on an interest free credit card, then you can make a very nice saving - £649 for 2 years service and the 16g iphone 5 (£529 from Apple).
My girlfriend just did this, but is paying off the phone at £50 a month (on a 0% card) so will be clear in 10 months, with just £10 a month to pay.
Also, it's just a rolling 30 day notice period - no 2 year contract.
http://community.giffgaff.com/t5/Contribute-Innovation-Promo...
Edit
By the way £10 per month for 2 years is £240
Outage information (in addition to the O2 problem): http://www.theregister.co.uk/2012/03/16/giffgaff_down_again/ http://www.theregister.co.uk/2012/01/19/giffgaff_outage/
£10 goodybag (change) 250 UK Minutes Unlimited UK texts 1GB Mobile Internet (Tethering will be allowed)
or
£12 goodybag (new) 250 UK Minutes Unlimited UK texts Unlimited Mobile Internet (Tethering will not be allowed)
Not as brilliant as they were, but still not bad. The only outage I saw was the o2 outage - was there others?
EDIT: just spotted their earlier one caused by the burst water pipe. Interesting how they spin the outage, as 'too expensive to prevent': http://crave.cnet.co.uk/mobiles/giffgaff-outage-too-expensiv...
Admittedly the deals aren't as bad in the states if you are maximizing your usage of the plan, but for my use case it's a total rip off, as are the UK contracts to a lesser extent (because of competition).
If you do go for the contract don't forget to call the day after it expires and switch to SIM only level prices otherwise any savings will disappear the the next month or so.
My current iPhone 4S contract is only 12 months which was available only to existing customers and worked out substantially cheaper than other purchasing options comes with an insane number of minutes 900 per month but was actually cheaper over the period than the 200 minute ones which would have been plenty.
In the UK for instance I definitely save money by buying my own handset. I use mostly data on my phone and pay £10 ($16) / month for my service. Contrast with the US where I was forced to have an $80/package to get any combination of voice/texts/data for my unlocked phone.
Even if I was taking the subsidy like a good American I would still be getting shafted to the tune of $1000 over a 2-year contract.