On one of our trips to Spain, we stayed in an apartment, booked at booking.com.
We came in close contact with the owners, and he could not stop talking about how much money he had to pay to booking.com for each booking. To make some money he had to raise the prices. So, eventually, the customers pay for the commission and booking sites like booking.com make lots of money.
I heard this more often on other trips later. Other accommodation owners asked us to email them or use their site for future stayings.
At Easyspain.nl, we try to keep the prices as low as possible for the accommodation owners so that they can lower the costs for their customers.
So we are starting with an annual fee of € 15 for one listing.