I find pricing £X amount per window doesn't really work. What about those ground floor windows that require A-Frames to reach? Houses where you have an assault course to get to the rear windows. Those that require a long walk to get to the rear. Others that you have to drag your ladders over garages and under electical cables to gain access to the back.
For me it's a combination of:
How affluent the area is.
How long the job will take.
How desperate is the customer.
What do I judge I can get away with.
A recent customer who I priced at £20.00 per month (for a large house with conservatory - about an hours work) told me her last window cleaner charged her £12.00 per month.
I asked her what happened to him and she replied, "I don't know, he just stopped comming". No wonder at £12.00 every six weeks. (I only do monthly).
If I charged her at £1.00 per window, it would be more like £40.00 per month and I'm sure she'd tell me to "naff off".
I'm happy with £20.00 per hour. The houses I feel like I'm being 'ripped off' (even though it was me who gave the price) tend to get dropped as I find it less stressful to bin them than put prices up (a character fault I know).