As Malc said, don't give it away. Turn it around and make a success of it.
Customers need to be educated in the ways of window cleaning.
Before giving up on the work, explain to your customers what the new price will be, and what you do for said price.
I am guessing that the previous guy cleaned the windows, but not the frames. If this is the case, sell your customers the idea that they will be getting the frames and sills cleaned as well. People need to know that they are getting value for money, so by selling them a superior service you can justify the increased price on this alone. Don't panic if some still cancel, but knock some neighbours' doors and try to fill in the gaps. As you get to be known in the area, you will get some of the cancellers back when they see you do a good job and there's no-one else to ask.
Best wishes,
John.
And remember: They will need you before you need them, as they will have more difficulty finding another (good) w/c than you will finding good customers.