Charging for services : value v cost

A guy wakes up one Sunday morning to find the kitchen flooded. Water is spurting everywhere and things are getting ruined. 

He tracks down a plumber who arrives to a fraught family scene. ‘Thank god you’re here’, says the dad. 

The plumber surveys the situation, pulls a tool out of his bag and stops the burst immediately. 

‘That’s fantastic’, says the guy, ‘how much do I owe you?’

‘¬£100’ says the plumber. 

‘¬£100?’. says the dad. ‘It only took you two minutes!’

‘No’, says the plumber, ‘that took me 25 years’