How much should you give somebody for putting on iphone screen?

How much should you give somebody for putting on iphone screen?

I'd ask them what they wanted, and that would be it. But I'd want to know before they touched my phone if they actually knew what they were doing, because if the phone does not work when their done, how do I get it working again? A business does this, they fix or replace it, if a "buddy" does this, it's opps sorry.