I would do it per house. Don't know about your area, but I would say, what £30. It is specialist as only our poles will reach. Anything up to say £70 in a good area. Not my thing. I look at it as how hard Is it? bungalow is a piece of cake and takes a few mins. Less work than doing windows. Houses are harder. Don't forget its not windows though and the people on benefits with a ladder and squeegee cant do it 
What a prat...'people on benefits with ladder and squeegee can't do it'.... I wouldn't get too carried away with your imagined superiority - you're still just a window cleaner!
And before you jump to any conclusions,I haven't got a chip on my shoulder because I'm predominantly wfp as well,although I still also do some trad.
We all know of,and get frustrated by the fairweather cut-price benefit claimants but that doesn't mean everyone on a ladder is. But what I find most objectionable is the implication that trad cleaners are inferior and treated as if beneath contempt.Sad.