How to get out of new AT&T contract and get discounted iPhone 5S?

So when the iphone 5S is released to the public, I want to get it at AT&T's discounted rate provided I sign up for their 2 year contract.

But how much would they charge me if I cancel a day later?

For example, IF the iPhone 5S cost $299 with a 2 year contract, how much would it cost be total if I canceled the contract but want to keep the phone?

The amount is usually based on how much is left on your contract.

Canceling after only one day will have a fee over $300.