How much would it cost for me to upgrade my broken 4s to get a 5s?

Today, the power button on my 4s stopped working suddenly. I've had this phone for less than a year, so I'm assuming that it's still under warrenty. It has no other physical or internal damage, and has always been in a case. I have an appointment at the Apple Store tomorrow afternoon, and don't know what they will do to fix it. If they replace it, will I have the option of paying the difference to get a 5s? ANd if so how much will it cost? Thanks.