With all due respect to aries. This is a straigtforward halacha question -see rama Choshen Mishpat 185:10 and chochmas shlomo 185;1) Generally speaking, anyone (proffesional or non proffesional) who redds a shidduch and the couple gets engaged is OWED compensation no different that a person who preforms any other service. The rate of compensation will depend on the custom of the places where they live.
Feedback from shadchanim indicate that customary shadchanus is between 1,000-1,500 each side. This is not set in stone. Families that are very well off typically pay more (and this would be a halacha shayla as to whether this would create a actual obligation to do so). I think in out of town communities the rate is a bit lower.
If a shadchan is known to have a set “fee” that is different than the standard rate, then by using them one is reeing to their fee. If a is not known to have a set fee, then that shadchan can not ask/expect more than the going rate. (what is the thresshold for having a known fee, i don’t know, but i do believe it is quite rare for shadchanim to have set fees. The fast majority (and all that i know of) do not have any set fees.
In a situation where it is clear that familes can’t afford the standard rate a shayla should be asked as to whether there is assumed mechila on a portion of shadchanus owed)
If a person (shadchan) explicitly states they are doing it free of charge then of course nothng is owed them (and the shadchan can’t change their mind afterwards).
For some reason there is a belief on the street that compensation to a shadchan is a nice/touching thing to do etc. What we need to realize is that it is a 100% bonafide monetary obligation that (in thoery) a bais din can enforce.
Generally speaking, Shadchanim who don’t receive the standard shadchanus (or anything for that matter) typically turn the other cheek and – though they are not happy – they realize there isn’t much they can do.