My DH used to be a window cleaner. It is a lot per minute but you have to remember nobody wants there windows done in the rain and at other times it is too dangerous to work, snow/ice or just simply too cold.
I think we pay £7 for two bays and one ordinary window, a light over the front door and two/three windows at the back, there are three windows he can't get too so I think they will just stay dirty forever!