Thats no cheaper than anyone else, you need to pay a huge chunk.
Tesco Mobile will certainly hope so. There are two standard tariffs for pay monthly mobiles. One is £20/month, fo 12 months and giving £60 of calls and textsand unlimited web browsing.
You'll have to pay £222 for the iPhone 3G 8GB, £320 for the 16GB iPhone 3G S and £407 for the 32GB 3G S if you want the £20/month tariff.
The other option will see you pay £60/month over 24 months for unlimited calls, texts and web browsing. You'll be able to pick-up the 3G 8GB and the 3G S 16GB for free on this tariff, while the 32GB 3G S will cost you £50.
There's also a pay as you go option, with the 8GB iPhone 3G setting you back £342, the 16GB iPhone 3G S costing you £440 and the 32GB iPhone 3G S yours for £538.