just a point, and I know people may disagree, but rather than walking away from those customers, or charging even less, why not reduce the frequency of the visit. Go every 8 weeks, so if you are charging 10squid, it worth 5 squid a month to the customer. And you win as you keep the customer, and you dont have to work your nuts ofd for silly money.
I think if you do a good job WFP, the windows will stay clean anyway.
Sorry if anyone does not agree