Erik is under contract with a textbook publisher to provide interesting and current examples to reinforce key theoretical concepts. He makes \(€ 50\) per hour for doing this work. Erik also wants to repaint a number of rooms in his house. A professional painter estimated the cost of this work to be \(€ 500\) (including the paint) and guarantees it will be completed in eight hours. If Erik instead paints the rooms himself, he believes it would take him at least twice that long. What is Erik's opportunity cost of painting the rooms himself?