I didn't know how marketing agencies make money either. But then I was told that there is an option where the agency and the client agree on specific goals, such as increasing sales or attracting a certain number of clients. If the goals are achieved, the agency receives a certain remuneration. This is a model that usually requires good analytics and monitoring of results. But perhaps the most common is the fixed fee. The client and the agency agree on a specific amount for a certain amount of work.
