Cheapest place to but iphone 4s?

My son has asked for a iphone 4s for his xmas and not sure where to get one from - he is 12 and on a monthly contract with Orange at moment but its only £7.50 a month with little date so not sure whether to buy outright or just get a contract - please any advise would be great full

I would just buy one, you can get them for like £100 now off gumtree etc

Gumtree or ebay

You should get it outright on either Play.com or try to find a good deal on gumtree.com

Hope this helped,
Siva.