How much would it cost to buy the iPhone 4S before my contract is up?

I have the iPhone 3GS right now but it's messed up. The home button sticks the ring\silent switch is glitchy and I just recently cracked my whole screen. How much would it cost me to upgrade to the iPhone 4S my 2-yr AT&T contract expires in January but I don't think my poor phone will hold up that long. Oh and if I do upgrade early will that count as my 2-yr contract upgrade or will I still qualify for a new phone in January because I really want the iPhone 5 I just can't afford it right now.

It would be market price. So around 500 dollars depending on the gigs.

I don't know if At&t does the same thing Sprint does, but if it does if you're upgrade isn't avaible yet, on their website they should show what the price is for your phone on your 2-yr contract

Call them and see if you can extend it 2 years on top of your current contract and get the new phone.

You can recycle old phones here on apple's site and they will give you store credit for doing that. Yours is cracked so obviously not much, but something is better then nothing.
http://www.apple.com/...recycling/