kako,
You’re not taking into account all of the work that goes into shadchanus, often hours upon hours. Also, since a shadchan does not pay for a shidduch which does not end in marriage (or engagement, depending on the community), the compensation for a successful one may be higher than for other services with a different system (e.g. a defense lawyer who gets paid even if his client is convicted) or success rate (e.g. a plumber who is able to properly fix the problem 99% of the time).