You could mention to the cheapskate customer that you didn't realise the chap who quoted them cheaper has been let out early on bail from prison as he was put away for several years for burglary and theft and remark what an absolute nutter he was when he went inside;D
Personally I wouldn't do that house that cheap anyway!
My customers tell me I'm more expensive than previous window cleaners before me but I come highly recommended to them by word of mouth recommendations as I'm trustworthy, reliable, honest and do a good job. The decent customers prefer someone reliable they can trust!
Tell them you have your set pricing structure for cleaning windows and you're not prepared to lower your prices, miss out windows or lower your cleaning standards to compete with new w/cleaner poaching your work. If customer wishes to go with someone else that's business, most of the time they will come back and you can up your price with them for not being loyal, that's life