our main business is one off letting cleans. we find the easiest way to price a job is to split it down into sections. for us this is oven/hob/extractor cleaning, carpet upholstery cleaning, window cleaning, general cleaning (ie kitchen,bathroom,woodwork cobwebs,washing down walls etc), rubbish removal.
if you give a seperate price for each section of the clean it also helps explain the large amount of work involved in a one off and will justify the price.