So if you're thinking about getting an iPhone 4S, you may be thinking that actually signing up to a 24-month contract with an operator just looks too expensive.

It turns out that's not the case: once you calculate how much the contract costs if you add on the pure cost of the phone (which can be found on the online Apple store), then you soon find that the contracts are quite expensive.

From our calculations, based on the prices so far announced (which doesn't include O2; its prices aren't yet available), the best deals generally come from Three. The most expensive ones - generally - are with Vodafone. OF course, you'd need to compare elements such as data, texts and minutes - which we have in our spreadsheet below.

But here are the pictures which compare the costs of the phones. The cheapest phone and contract overall: Three, for the 16GB iPhone 4S, which costs a total (over the 24 months) of £849; that's for a phone which costs £499 unlocked, on a contract of £30 per month (which works out to an effective saving of £370 on the total cost. To get the same from buying just the unlocked phone, you'd need to be on a contract costing £14.60 per month.

Here's the graph for the 16GB iPhone 4S:

And now here it is for the 32GB model:

Finally, for the 64GB model:

And because it's more complex than it seems - and you need to make careful comparisons between the different offerings based on what you need in terms of minutes, texts and data. (The data is very important on a smartphone, of course. For most people, 500MB is plenty in a month.)