Actually - the #s are different for different posters. So there's no hard and fast rule on how much it would actually cost to pull of this "ditch"
But whatever the cost - it's not all that different than buying the phone outright without a contract. AND you don't jeopardize a relationship with AT&T should you want to return for whatever reason within 6 (I believe that's the magic #) months.
Why not just buy it outright and have all the freedom you want vs trying to ditch?
Because to most people, ~$150 far outweighs the opportunity to use AT&T in the next 6 months.