Will sprint unlock my iphone?

I'm getting an iPhone 5s from a friend and I want to unlock it to use it with straight talk. Will they do that? How much will it cost

No they won't. They unlock for no one. Not until Feb 2015 when rules change that they have to. Even if your uh 'friend' is the original owner, he'd have to do it in Feb 2015 and not you. You'll have to buy an unlock online. They're 50/50 at being high priced scams or actually working. Also note, until the new rules it's still illegal too. Best luck.