If you are going to a shachan, then ask up front what their rates are. If someone you know is bringing the shidduch to you and they are NOT a professional shadchan then you can either ask them what they would like for shadchanus or ask your Rav what to give.
We have discussed this issue here in the past and it got very complicated. There were those who considered themselves “shadchanim” since they made many shidduchim but did not really put out a shingle and expected the “going rate” of a professional but did not “invoice” the baal simcha and was very upset when she did not get “paid”. Rightly so. Although it is the responsibility of the Choson & Kallah to pay the Shadchonim before going to the Chupah, a professional shadchan would have a written contract and therefore there would NOT be any misunderstandings. BTW, a Chosson Rebbe and a Kallah Teacher will usually discuss this and remind the C&K to make sure that the shadchan is paid up before the chupah.
If you are in doubt what to pay, you can ask your Kallah Teacher, since you would most likely be taking classes in your local neighborhood and the “going rate” would be whatever is right for that neighborhood. What is very important to remember is the Shadchan MUST get paid before the chupah. There are many inyanim brought down about fertility problems and shalom bayis issues if the Shadchan doesn’t get paid.
Whatever the rate is, I am sure that everyone here wishes you much hatzlocha in find your zivig and may you be zoche to pay the shadchan!