If you want a serious answer, it really depends what you consider a low price. You can get them second hand from probably mid to high 300s upwards on eBay. Brand new, sim free, they'll cost you £529 I think no matter where you go. Your best bet is to go with a contract. All the networks will offer you various contracts where the handset will be yours for between £0 and approximately £130, but you'll then have to pay a monthly fee which will normally be at least £30 and for at least 12 months. If you're prepared to go for a second hand iPhone, head on to eBay and get bidding, you might get lucky and get one for mid 300s. Don't go for an offer advertising a brand new sim free iphone 5 for a very low price, use your common sense! Unfortunately my friend I'm afraid the iPhone 5 is an absolute top of the range mobile phone, and Apple things don't tend to go cheap. Contract or no contract, the reality is you'll be shelling out hundreds of pounds if you want one.
Have looked round internet for iPhone (and purchased one new) before.