How long does it normally take you to earn £100?
How long have you spent agonising about this so far?
What sort of discount would you look for if a tradesman working for in your house made a cock-up on a) a £200 job and b) a £700?
How have you reacted when you've broken something, perhaps, and the client has gracefully told you it didn't matter and you needn't recompense them?
Yes , it's galling but we all suffer occasionally.
Put it down to experience.
Rog