price per window, area does matter, the further north you go cheaper it tends to be, i've found it depends what their previous cleaner has charged, you might get a string of quotes where previously they were underpriced, or even better, overpriced.
I usually go on the basis of 80p-£1 a window and usually works ok for an hourly rate, and gets the customers.
I'm based in your area but as we know, some parts are more afluent than others