Switching from US Cellular to AT&T?

I have the htc wildfire s from us cellular and I'm planning on getting me a black iPhone 5 from AT&T next week because I want a new phone. I'm not in a contract with my current phone, so how much would it cost to switch carriers and upgrade to an iPhone 5 16GB?

Ya can get an iphone 5 for 199$ but I suggest a 4s it's basically the same phone and its cheaper and its accsesories are cheaper.