I know exactly what mean. It would be almost impossible to be a true advocate for the client if you were being paid a sales commission from the contractor. The commission I was referring to is more like maybe a “finders fee”, if you will. I run across all sorts of projects and not all of them are appropriate for a designer. So I refer these jobs off to my list of contractors. If they get the job, they send me an appropriate “finders fee”. If they refer a design only job to me and I get it, I send them an appropriate finders fee as well. It’s not a lot of money individually, but it keeps us all interested and it can be surprising how fast it adds up.
As a designer I operate very similar to Roger below. My design fee is separate from the project management fee. The project management fee amount changes depending upon how much supervision and planning is needed.
On the other hand, there is an LA here that requires all his contractors to pay a 5% fee if they get the job. I have worked with him as a contractor before and I can tell you I was happy to pay it. He brought me work I wouldn’t have had the opportunity to bid on and his designs were very good and easy to bid from.