How much would it cost to upgrade my iPhone?

I currently have an iPhone 3GS from at&t. I have had my contract since december of 2011, and I'm pretty sure the contract ends December of 2013. I want to upgrade my 3GS to an iPhone 5 when it releases, but was wondering how much that would cost(estimation). Do you think there would be some type of deal on the iPhone 5 for people who want to upgrade? I've never upgraded a phone before so I don't know how this works.

$200. You will be able to sell your old phone for around $35

If you are not yet eligible for upgrade, you will have to pay full retail, which usually for iphones is $600-$800+