Sooooo......
They needed cleaning...?
The last window cleaner charged a whopping £6.........!
My reply would have been either, so when did you last see him ( usually at this point I've been told the last guy, for some unknown reason keeps failing to turn up

) or well, you get what you pay for, thank you and goodbye

Your price if anything, sounds cheap, unfortunately in this (service) industry a percentage of people will always try it on or think you should be working for next to nothing, they themselves may only be paid £7 to £8 ph paye and think giving you £6 is a fair price, but as we know being self employed is far more than the time on the glass.
From experience if you want to say anything, then keep it calm and not directed at them, like "your neighbours have found my prices fair and the service excellent" but overall these ignorant types will never be persuaded by any justification so don't let it bother you, don't show them it bothers you, walk away knowing your best off without this type of customer
Darran