How calculate Property Management fees for empty homes?

[Edgar Garnier]

Hi everyone, 

As a property manager we managed and rent several properties for many of our clients, but we are having more investors buy properties and keep them empty, well, they assume they will use them a few times per year. We normally we calculate this based on the rental price, but properties as they are empty and the price are above 1Mio on market value, It seems not fair to charge based in the rental value. Any input? Many thanks