well, it's not about my expenses related to extra person @Gordon0, it's about value of extra sleeping space.
i can explain this better with an example:
i'm renting 2 bedroom+living room condos, in the bedrooms there are space for 4 guests, this's included listed price.
there's also 1 sofa-bed in living room so if they want to book 5 guests, 5th one is an extra
rates for 4 people are
around 100usd/day during summer season
around 40usd/day during low season
so extra person fee has to be 20% of daily rate which should be 20usd during summer and 8usd during low season. But it's not possible because of fixed price so if i designate it according to summer season as 20usd, it's too expensive for someone who wants to book for low season.
all major OTA's are using a percentage system linked to daily rate but not Airbnb unfortunetaly.
ME