If you did not purchase AppleCare Plus for your iPhone, then you are out of warranty. In that case, you will need to pay about $229 plus taxes for an out of warranty replacement from an Apple Store. If you purchased the Apple Care+, then you should probably get your phone replaced for free.
I purchased Apple Care+ on my iPhone 5 when I bought it on launch day, and when my battery started to bulge a month or so ago, Apple replaced it for free for me. I walked into an Apple Store and 10 minutes later walked out with a replacement iPhone (although I did make Genius Bar appointment ahead of time).