Why don't you tell her that you'll do it for something daft like fifty quid?
You never know.
Some tight gits don't have their windows cleaned for years then when they decide they want to sell their house; they then get desperate for a window cleaner who'll charge 'em seven-pound-fifty for a one-off clean.
Estate agents actively encourage filthy buggers who want to sell their house, to have their windows cleaned, so price high!
I think out of the last five I've been asked to do as a one-off, I only cleaned two of them. All the rest just looked at me like I was daft when I said 'thirty quid for a one off... etc' for a normal small house.
Still, it's no skin off my nose.