I never hear there is a “certain amount” but rather that shadchanus should be SOME amount of money preferably. There is such an inyan, and it is for the mazel of the couple that the shadchanus should be paid.
How could you be mechayeiv a poor parent to pay a “certain amount” to a shadchan? Whatever amount it is, should be accepted graciously. If someone is a professional shadchan, then he or she should be upfront about the amount that is expected, as this is their parnassah. But to a relative or friend?????? pro or no, I cannot imagine they expect “real” shadchanus. My friend recently made a shidduch for a relative and they gave her shadchanus (unrequested by her). She gave the money to the couple for a wedding present.