I charge by the sq ft, usualy 17.5 pence but this can vary a lot depending on area, soiling etc.
Minimum job charge £35, but if around the corner £25 for a small room.
The thing to remember is you do not know how much a customer is prepared to pay!
Some customers are looking for a cheap price, if you look like a cheap company that is what they expect, if you look like a professional company price isn't as much of an issue.
I once went to a lady who is now a regular customer down the road and cleaned a 10 x 10 living room for £25. When I quoted her at first she said I would have been prepared to pay £50 to have it cleaned
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I now recieve £350 worth per year from the same lass, but thats another story
John