If you agree to do a one-off, make sure you price it at least double what you would charge for a normal clean. At least! Check it first too, if it's covered in paint splatters, green, hasn't been done in years, go a lot higher.
It's rare I do a one-off clean because I charge well for them, so most decline.
Today I was asked twice; some bloke just said, "Can you do number 19, tops only", I said, "Yes, but my minimum price is £20!" (It would've been for about four windows; dormer bungalow) and he declined. Another said - after I gave her my price - "I have to speak with my husband first!", and I never heard back.
But I know if I charge cheap, I'll regret it, so I don't.
As per what the rest said, you'll have to be determined and stick at it.