After seeing the "I don't think giffgaff works out cheaper than contract" comment (or something along those lines) posted by different people and growing weary of always answering with the same response i.e. "You have to look at the total cost over the life of the contract etc etc" I've created a cost comparison calculator!
It's in Microsoft Excel format, and you simply enter the per-month cost and length of the contract you're looking at (or have been offered by your current network) alongside the cost of the goodybag you'd be buying with giffgaff - there's even a field to enter the 'up front' cost of a phone if you're thinking of going SIM Free. The calculator will then show you whether you will save money or have to pay more by being with giffgaff (and the figure).
You can download it here: http://tinyurl.com/ggcalc1
It'd be great to hear your feedback, and if you wish to refer to this post in response to any new questions you see in the forum, that'd be spot-on.
If you find this useful, please give me Kudos by clicking the star icon 🙂