Picasso did this once : one day he was relaxing at a park, a woman recognized him at asked him to draw her portrait. Picasso quickly scribbled something and before giving back the drawing, told the woman :

"It will be 300$, madam."

"That's way too much", the woman replied, "it took you only 10 minutes to do it !"

"That's where you are wrong madam, it took me all my life."