DY, would you hire a contractor under the same premise?
No. But I would a shadchan, because they can only expect the going rate, not more.
I think there is an inyan of gneivas daas when you are not told in advance what to reasonably expect.
You can think what you want, but that’s not the halacha.
A shadchan could charge ANYTHING then, after the fact.
Also untrue. That error explains much of your misunderstanding of the entire issue.