Showing appreciation is mentchlechkeit 101. The question really is, can you dictate how the appreciation is shown. One person might feel $100 is a nice way to say thank you, while another might feel it is too much. While another wishes they had the $100 to feed their family. The ways to show hakaras hatov are as many as their are people.
If a shadchan has a fee for services, then in a free market you can pay for the service at the set price, try and negotiate down the fee or shop elsewhere. Whatever floats your boat.