How much for battery replacement ?

Depends on your location. It's $49 plus tax in the United States if performed directly by Apple at an Apple Store, but can be different in other countries and can include different fees including shipping and/or diagnostics. For instance, I read about someone who was trying to get it done in the Philippines, and the only way was through an Apple Authorized Service Provider (where they're free to charge diagnostic fees and set their own prices for the service). This one had a mandatory diagnostic fee (even if they didn't change the battery) and I believe altogether it was about the equivalent of US $160.