AZ, Professional shadchonim take big bucks and they are entitled to it. Parents who go to them do not give them a “gift” when a shidduch goes through they give them a payment for their services and that can be up to and beyond $10,000. If that is not enough of an incentive for the Shadchan to do their best for each and every client, why would the addition couple of hundred help? No one sets the shadchan’s fee but the shadchan.
On the other hand, non-professional shadchanim who do it for the mitzvah still have to get paid and are usually grateful for whatever gift whether monetary or otherwise that they receive.