No different than if one hires a plumber or electrician and doesn’t make up ahead of time.
Al Pi Halacha the homeowner is obligated to pay the going rate and if their is a range the homeowner is within their rights to pay the low end of the going rate.
What i think some of the posters are having difficulty grasping (myself among them) is why this discussion seems to always center around shadchanim and not plumbers/electricians/dentists etc….. who provide a service and don’t state up front their charges.
Surely if shadchanim stated a fee upfront this would alleviate this issue, but their reason for not doing so, is because it is their perception that by doing so they would be looked upon askance by a community who that has unfair expectations of them.
(just look at some of the posts in this thread, and i think we can agree that those comments would never be made expected of plumbers/electricians etc).
The overwhelming vast majority of shadchanim (even the full timers) don’t look at themselves as in shidduchim for parnassah-although if they don’t end up with some kind of a parnassah for their effrots they obviously won’t able to continue to devote the immense time and commitment that it requires- and thus they are uncomfortable stating fees upfront.
However, that in no way shape or form changes the obligation to compensate them according to the going rate.
If the broader communities attitude was that shadchanim should be and deserve to be properly and fairly compensated I think it would be more likely that we would have shadchanim feeling comfortable stating or letting it be known their expectations up front.
but we are far from that point…..
For the record i am not a shadchan.