And just where did they get these figures?
Two years ago I needed to put a roof on a rental house and considered hiring & supervising casual labor to do the job. I wanted to stay legal so I called up the state and asked about workmen's compensation insurance. The answer I got was that it would cost $11 per hour. Since the laborers wanted $12 per hour this is $91.70 per $100 of labor.
I gave up on that idea and hired a roofer who charged about $25 per man hour for his crew.