It seems like I'm a bit late joining the party, but I feel the need to comment ..... seeing as I'm a window cleaner :)
Short answer is - it depends on where you live; what kind of house you live in and does he use ladders, or a reach and wash pole-fed system??
I'm NW England, use ladders and here's a few of my prices:
2 windows at front of house (one top and one bottom window) + fanlight over the door = £2.00 - £2.50 per clean.
Standard terrace house, consisting of 2 windows at front, 2 top windows at back, 1 living room + 1 kitchen window = £4.50 per clean.
Standard detached / semi detached (conservatory not included in price) = £5.00 - £10.00 depending on size of house and amount of windows.
My biggest house is £30.00 and that's for a nice BIG posh house in its own grounds .... electric gates, separate annexe .... you get the picture??