@Johan2252
That varies, but I have generally seen $50-$100. However, Airbnb only allows you to apply a fee to the entire stay; not per night. This means if the guest books 1 night they pay the same fee as a guest staying 30 nights with a pet. For this reason, some hosts add a pet fee per night/pet in their house rules and collect the addl fees using the Resolution Center. Be sure to limit the number of pets you want as Airbnb defaults the number of pets allowed to 5. It's found under House Rules:
You can also look at other listings in your area and see what they are charging and what rules they have for pets in House Rules. You can search for listings that allow pets by using the pets allowed search filter.