Can you buy an iPhone 4s from AT&T without renewing your contract?

And if you can how much would it cost without renewing your contract

Yes

look at the non-contract price on their website

If you purchase the phone yourself than you could just add it to your plan. As far as cost go on the website and look up the particular model you are interested in and the prices will be listed. Make sure you buy a phone that works with AT&T.