Need new Iphone but in middle of AT&T Contract?

My Iphone 4S was lost or stolen. I went to Best Buys and they said it would cost $600 for a new because I was in the middle of my contract. I went to AT&T, and they said I could purchase a ghost line to my existing account for an additional $20/mo for 2 yrs plus the cost of a new Iphone ($100-200) depending on Iphone series. I do not want to pay more then $200 for a new Iphone. Can anyone offer other suggestions? Wants: Must be Iphone 4S or greater and I want to keep my old number.

Added (1). It would be $250 to buy out my contract. It was my understanding I could keep my new number with ghost line. Will they let me cancel my contact and buy new phone with existing phone number?

At this point you can see if you can get a used one on craigslist, used iphones are popular there because many have upgraded. Thats an option at least until you finish your contract then you can look at other options.