If he's only charging $20-30, he's only replacing the glass. The Genius Bar is probably replacing the screen and glass since that's an easier procedure for them than just the glass.
I think the screen is one unit and you can't just replace the glass on the screen. It is a very quick procedure for my friend too. A couple of screws and a ribbon connector and away you go.
The parts are cheap on eBay. So far the $3 battery has been a dud though.
