How much does it cost to break an AT&T contract?

I got an iPhone 3gs 22 months ago on a 24 month contract. I want the iPhone 4s, and I can't wait 2 more months. I'm having so many problems with this phone and I don't want another 3gs. How much would this cost, if the 4s is $99.99 with the upgrade?

Go to at&t and explain the trouble you are having with this phone. Say you want to pay the 99 bucks to upgrade to the new phone now. Say if no you will go to verizon or sprint and you won't pay your bill. They won't want to lose you as a customer.