As another user replied, hotels have a cleaning staff that they just pay a salary to - it is, for the most part, a fixed cost. The owner of a single rental/room really is usually paying a flat fee every time they need their placed cleaned, and thus can have widely different costs based on whether someone stays 1 night or 28. They simply don't have the luxury of averaging their costs over a wide base, no matter how many times you put that word in italics.
This is maybe the least complicated business problem ever. Businesses with fixed asking price but variable cost do it literally every day. This is how it is done: you look at your records and figure out average length of stay, giving you your average cost per customer. You can base your price added to the room on that cost, plus say 20%. Hiding the cost in tack-on fees is not necessary and it's clearly just a method to increase visibility for your property and add the real cost in the back end.
Oh right, averaging.