You have to set a minimum price, or you end up having these silly little jobs that waste your time!! I set a minimum price a long time ago of £10, and because of that I have several great little jobs.. ie I have a barn type conversion with 2 of the smallest skylights you have ever seen, I clean just the skylights every month for £10... if I priced it per window I would be getiing 2quid, which would not be worth bothering with.