oomis: the one error in you post is “IMO” as what you write is against codified halacha, your opinion doesn’t really matter on the issue.
Perhaps what you don’t know is that even after the shidduch is completed the overwhelming vast majority of shadchanim don’t state a fee.
The discussion is simply what are the two sides obligated to give under the circumstances. If a shadchan completes a shidduch and if (as is the common case) that no set amount of shadchanus was agreed upon, then how much must each side pay.
just like if one calls a plumber or a electrician and no fee is agreed upon prior to the work, the plumber or electrician is certainly entitled to get paid after the job is completed. and his pay will be determined by the going rate. (if the service provider provides a rate that is within the range of the going rate and they can substantiate that such a fee is that specific service providers standard fee, the halacha might be that he is supposed to get that as opposed to the homeowner stating i will only pay the low end of the range. of this i am not certain, and one should ask a qualified poseik).
And if the shadchan does state a fee -after the fact- that is beyond the range of the standard practice in the given community, or even if the shadchan requests a specific end of the range, the shadchan’s stated fee is irrelevant, UNLESS it was well known what that shadchan’s fee was.
In such a case the two parties who used that shadchan are al pi halacha considered to have agreed to the fee of that shadchan being that it is well known, even if the shadchan didn’t tell them specifically and (i think the following is correct althought i’m not positive) even if the actual parties claim and or actually were unaware of that shadchan’s fee.
however this kind of case is not common. As the reality is that shadchanim don’t state fees ahead of time, and the overwhelming majority of them don’t state fees after the fact either. They rely on the parties to (hopefully) do what is right and proper and the shadchanim probably hope that the parties will be even a bit generous.
I for one would wish (as i think yo would) for it to become standardized that shadchanim state up front what the “fees” are if a couple gets engaged. However the overwhelming vast majority do not do so, and their reason for not doing so comes from “aidelkeit” not from a cruel plan to hold up the sides.
In fact, if the (horrible/mean/money hungry) shadchan wanted to pressure the sides to “milk” the most shadchanus they could, the worst time to do so is after the shidduch is completed. At that point in time the shadchan has no leverage. Their best leverage is when the singles come to meet them initially, but as stated above, shadchanim don’t do so because they aren’t comfortable doing so, and the primary reason they aren’t comfortable doing so should be self evident from the way shadchanim are perceived in these posts.