No fee...or can be accused of discrimination..

Pet fees

Service, assistance and emotional support animals are not technically pets and owners do not have to pay pet fees. The landlord, however, can charge a security deposit and may still seek money from the tenant if there is any damage caused by the animal to the home. Also, if there is a nuisance issue the landlord does have the right to try to remove the animal through legal proceedings.

 

More here: https://www.hud.gov/program_offices/fair_housing_equal_opp/assistance_animals