The difference between a plumber/caterer/musician and a shadchan is that the other services state up front what they charge. Imagine telling your caterer that you’d like a steak dinner, and at the end of the event being presented with a bill for $300/person. It wouldn’t fly.
So, I agree with the poster above. The shadchan should either state his/her rate up front, or should not expect any sort of payment, and graciously accept whatever (if anything) is given.