£1.00 per window works for me (that's £1.00 per window, regardless of size; it generally averages out nicely; (much more for 3+ storey work), but if there's any issues such as the customer looks wealthy, add a bit more on for 'Matt's rich tax', or extra for any access issues, and also don't forget to add some more if the customer came to you by reccommendation (taking into account how much you charge the person who reccommended you).
But you've got to use some common sense when using the above formula.
And as a rule, when someone says 'we can't get a window cleaner round here', there's normally quite a few of them.
Anyway, I quoted for and cleaned a lovely farm house this morning; I counted 31 windows; no access issues, pvc frames; a perfect job, so I didn't add 'Matt's Rich Tax', or any thing else on. It's in a place where we clean a £50 house, and four others all priced above £15.00.
The lady asked how much and I said £31, then thought about it and said, 'let's call it £30. It took two of us 30 minutes to clean it; on a first clean; so we should knock 10 minutes off that at least!
That's good by my standards; she seems like a top customer too.
Pricing is more of an art than a science. I also do a lot of terraced houses, all compact work, and charge them between £7 and £9 each and it works out well for us.