How much would it cost for my iPhone 4s?

I have AT&T and my friend gave me her iPhone and I would like to use it for my plan.
What would I have to do to unlock it or use it with AT&T because she got it from Sprint?

Technically that's not really suppose to be done at all and it may not be able to be used on that network. Since the signals for AT&T phone service is a different signal than Sprint is so there's different phone service chips install into the phones… That is why companies make phones specific for certain carriers.

You can get a third party iphone factory unlock service to have it unlock.

An iPhone 4s for sprint can work on ATT If you unlock it. You can unlock it by searching on eBay for iphone4s unlock code. You will have to pay the seller a fee & they will email you a code you can put into the phone to use it. Just remember, even if you unlock the phone, you will not be able to use it if the phone was reported stolen/lost or if the line the phone was used on was not paid in full. Good luck to you.

if you have more questions, feel free to msg me.