This would be like an 'economies of scale' principle applied to a rental; the larger (or in this case the longer) one stays the cheaper it becomes per unit (in this case per nights). Very interesting thinking. It may be tricky to handle on Airbnb's part, accounting wise (no longer a fixed booking fee, but a changing one) and maybe a bandwidth eater.
This give me an idea, but it still needs a lot of clean-up work. 😎