Nokia repair

Hi, I took my phone to vodacom in tygervalley. It was defective. While I was busy with my phone it just died. They sent it for repairs, and the repair centre phoned me today to inform me that the estimate for the repair will be R580. The girl said it was the battery. So please tell me why is it so much for this repair of a battery? Its not like I broke the phone or something. Nokia lumia was defective while using it. Do you have proof that there was damage on my phone ?? I really can't believe that it can be so much money just for a battery. Its not like money grows on trees. I would really want proof of this, because its unfair towards me. The phone is then still under warranty! And I have the phone insured, doesn't that apply? Please get back to me.