Cancel my line 30 days after my 2 year contract renewal?

[sonicrobby]

So my contact ends in a few months, and I realllllllly want to leave AT&T. I'm going to switch over to simple mobile. The thing is simple mobile doesn't offer phones. So I'm trying to find out which is cheaper, buying an unlocked iPhone 5 at full price (somewhere around 600-700$) or if it would be cheaper to renew my contract, get the iPhone 5 for 200$ and then cancel my contract. The issue, I don't know how it works. I'm an extra line on a family plan (in case that matters).

So how does canceling the contract work? Is it one flat price I pay? Or is it a price based on how many months left on my contract?