My friend wants me to change the break master cylinder,how much should I charge?

Depends on how good of a friend this is! Are you qualified and experencded enough to complete this 'brake' repair correctly? Know how to bench bleed the mast. cyl. etc?

If you r doing it as a friend, a cup of coffee and lunch. If you r doing it as a mechanic then be careful. There r alot of things to consider. Insurance is the main thing.