Sorry Dave but if you think you have to be cheap to keep the work then I think you are mistaken. I have quite a few houses like this and they pay for my services.
They don't question the prices as generally I have been recommended because of the quality of work not the price. I also charge a high rate because whilst I am cleaning this house which I would anticipate would take hour and half I could be elsewhere on smaller £20 hits which I would clean in the region of 6 an hour.
The one thing to bear in mind is generally people mix in similar circles to themselves and if you do a good job word of mouth counts far more than cost, if you're too cheap people will think your missing windows or not doing the job properly, am not saying this is you in anyway so please don't think I am, just generalising what people think