See sefer Taamie Haminhagim siman 938 (טעמי המנהגים סימן תתקלח). The commentary of Kuntrus Achron (קונטרס אחרון) has four seifim bringing down the various shadchanus fee in percentage of the nadin. It ranges from two to four percent of the nadin.
If there isn’t a nadin, there is also mention of a percentage reference to the amount the parents give to the children.
Short answer: last I heard, the fee was $2,500 divided as $1,250 from each side. Two years ago.