A few tips mate:
Always work some sort of pricing system before you go to any job to give a qoute.
First work out how long it will take you to clean a window.
-- for me one or two pains a minute tops.
Then decide how much you need to earn per hour to pay yourself a wage you are happy with.
Take into account traveling time, cost for petrol etc.
If it was six sqaure pains, no fuss on ground floor that I could reach, I think it would take me about 5 mins to do the job, say 5 mins travel.
I say £5.50, its a very small job, and they wont want to pay to much more the £1 per window which seems about average where I live, but your area could very very different in the way of price than mine.
But seriously, do more calculations and take everything into account setup time etc. Charging the correct price is very important to the success of your business. If you make the mistake of going too low early on you could pay for it in your own time latter on.
I always think there is a fair price thats both good for you and your customer. But you will always get ones that want everything for pennies, and dont want to pay you a fair price. Be blunt and tell them you wont clear enough to make a living and it wouldnt be worth your while to charge any less.
Secondly the market has a great deal to do with it as well, a pricing structure that gets you a good number of customers might not two months later because of the market and nothing to do with what you are doing.
There is a range between how much you can charge and still make a profit and how much your customer is willing to pay. You bottom figure should always be below your customers price, or you will be out of business as no customer will accept your price, and you cant go any lower or you will be lossing money, yes...
The trick is to give a qoute that is above your profit making minimum and not higher that what the customer is whiling to pay. Now here is your choice, you can go as low as you can and get allot of customers fast, but there may be another window cleaner doing the same work but being better of because they charge say 50p more.
Now whats better? well on one hand you will get customers easier than him, but because he makes more, he may be able to offer a better service than you and in the long run be more resiliant to fluxs in the market, because if times do get tough he can always lower his price but you cant, you follow.
So my advice, is dont be the dearest window cleaner, but dont be the cheapest, and get your price as close to your customers "willing to pay" price as you can. But remember when you get a customer that says Ohh I will only pay £5 for this. Walk away, because I dont mean what any one person is willing to pay, I mean the market as a whole.
So if you go to 100 qoutes and 80/90 of them are happy with your price, then you are doing good. If you get 100 yes's you might be a little low, If you get 50/60 yes, you might be a little high. Nobody can tell you what is the right price is because areas vary so much, and in a years time the right price could be totally different.
An idea might be to get a good window cleaner, van signage etc, call him up and ask him to clean you own windows and see if your price would be anywhere close to him/her.
good luck mate