Kako: you have to pay what halacha dictates not what you feel is the proper amount. Same for all halahic brokers/service provides. You know a plumber came and fixed my drain. Standard rates for his work are 100 but i don’t think he deserves it. I mean he only spent 10 minutes on the sink. I think i’ll give him $30 instead. $150 is outrageos for 15 minutes don’t you think? Well that’s just to bad and to not pay his fee would be a issur doraisa of “oseik schar sachir”
For situations where one person gives the name (and gets yesses from both sides), and then someone else serves as the go between, (this is common where the suggestion comes from a family memeber or friend and they would like to have someone more experienced take over, then i believe the halacha is that the “suggester” has the halachic status of a “maschil” receives 1/3 of the customary shadchanus and the person who took over receives 2/3.
If someone has a idea for a shidduch but isn’t succesful in getting yesses from both sides then there is strong room to think they don’t have the status of a shadchan at all (of course ask a shayla). If one person gets the yes from one side and one person gets the yes from the other side then it would seem they should share the 1/3. Of course all these and similar scenarios should be presented to a rov to pasken.