About comission rates

Hello everyone,

I am active booking.com user for one year.

I was normally paying 15% of comission of my reservations. 

I do not have a business property, am just renting only one room of my flat and not at all commercial.

I did not find quite just to pay 30% of comission for the reservations who are always less than 500 euros.

Is there anyone who face with similar comission rates?