AZ. Its a matter of semantics. You yourself are not discussing this from the truly altruistic middah of hakaras hatov. You call it hakaras hatov. I call it shmearing. We both agree that the shadchanim will remember those who “remember” them.
Regarding how much shadchanim ask for. 18 years ago when I was dating, I was recommended to a shadchan. This shadchan wanted 5 thousand dollars, from each side for a successful shidduch. I didnt use him (never got past that first phone call), because I couldn’t afford him. If he is still around and involved in shidduchim, I would imagine that his fee has increased somewhat with time.