How much is an iPhone 6 replacement battery?

My iPhone 6 battery is horrible especially after new update. I am thinking a new battery will help with the wear down time has done to the original battery. I see a lot of batteries with kits that are less than $15... are these safe to order for me to try to replace the battery of my iPhone 6 and not risk the battery exploding or having other problems? Anybody have experience with these aftermarket batteries and how much is it? I don't want to send my phone in to Apple, would rather take care of it myself.