i get this a lot as well, the way i look at it is i aim to earn £x an hour or a day an if i get that then im happy, personnaly i like this reply when i get it an i have had customers offer me more for the job but i tell them that the price i have given them is the price im charging, i find that this sort of honesty gets me more work. I love the people on here who say they earn £100's a day an charge £50 for a 10 minute bungalow, dont listen to them, just keep going the way you are an it wont be to long before you are hitting really good money each day you go out.
A lot of people keep mentioning £30 odd an hour on here, well if you can hit that for 8 hours a day thats £240 a day, an over 1k a week. I would love to see someone show me proof of that sort of earnings :
I charge £30+ per hour. If you want proof you could get a copy of my latest accounts from companies house.
£30 per hour is not that high especially if you are employing people to clean windows. Every cost that you have quadruples (at least) as soon as you employ.
Insurance: Sole trader: £100 With employees: £2500
Vehicle Insurance: Sole trader: £500 With employees: £6750
Professional fees: Sole trader: £500 With employees: £3000
The list goes on.
Plus there are costs that don't even exist for 1 man bands.
Fees for Contracts of Employment: Some folks pay £600

You can get them here for £150:
http://www.lilihunter.com/cart.htm 
28 Days paid holidays per person per year.
How many of you paid yourself for 28 days holiday last year? Not chuffin many I'll bet.
When you work in the hols that takes it down to £27.50 per hour across the year then there is also VAT to consider. That's you down to £22.25, nearly a third gone before you factor in overheads.
In conclusion £30 per hour. Any less is pointless and you will end up losing money.