For a first clean my extra charge ranges from zero to +50%. This depends on a number of factors - some of which are how much I want the job, whether I feel that the customer will be a good, reliable one, if I'm light on work etc.etc.
For a one off same applies except that it ranges from zero to +100%. If I've run out of work for a several days, someone phones for a one off, and I could do with a bit of cash flow, I might just do it for standard rates and use that as a lever to try to convert it to a regular job. I have managed to do this in the past and gain good customers from it. More often than not though, it does remain a one off.