If I already have a battery for my iPhone 5 how much would it cost for Apple to put it in for me

They will not do so. Apple does not sell iPhone parts. If you have not opened up the casing on your phone, and it's not physically damaged, Apple will install a new battery for you for $79.

There are no legitimate sources for iPhone parts. The only sources for parts outside of Apple, including batteries, are stolen, salvaged, or counterfeit parts.

Nothing. They won't put anything in that you buy separately. You need to take it in for service and pay the battery replacement fee.

You might get a free repair.

http://www.apple.com/support/iphone5-battery/