Want to upgrade to iPhone 5 from iPhone 4 without being eligible for an upgrade, what will it cost?

I have the iPhone 4, and I am not eligible for an upgrade anytime soon because one of my sisters used mine. I want to buy an iPhone 5 and then give my youngest sister my iPhone 4 but I do not want to have to pay full price for the phone. I can't seem to find anywhere on the AT&T website where it says how much it will be.