I also have a minimum charge of a tenner...but that is flexible to a certain degree.
Call out a plumber or an electrician and you will have a minimum charge, probably covering up to the first hour, and yes, I know, the circumstances are very different, but it makes life very much easier to have a minimum charge.
If someone comes up to me in the street asking how much I charge, or call up on the phone then I start out telling them there is a minimum charge of a tenner, and that it could also increase depending on how far I have to travel, if they don't want my services then fine.
As Dave has said, you have to cover administration, and even the smallest of jobs requires you to get there, unload, do the job, re-pack your vehicle, invoice or collect...
I don't care if they are all back to back, and fortnightly?? Even then, if all of your round was fortnightly, there are only so many houses you clean per day, so why clean them cheaper because they are fortnightly??
Time taken to clean them isn't sufficiently different from cleaning ever 4 weeks rather than every 2 weeks, I find little difference in 4 weekly to 8 weekly.
So I'd personally rather have higher priced work either monthly or 8 weekly, even if that means needing more customers and more travelling...but then I prefer a spread out round to a compact one...
Average price though?...mmmm.....well my minimum target price is approx a tenner, my average price for domestic work though is over £20.00.
Ian