Paying the Matchmaker

Question

LawrenceW is quite right that it is incumbent upon everyone to try and bring together suitable marriage partners, and friends are a significant source of introductions. In regard to the matter of payment to the Matchmaker, I don't know where this is brought, but I do know (having been told so by my Rav) that it is extremely important (apparently for the success of the marriage) to pay the Matchmaker for his/her services. I suspect, however, that it's the "how much" which has Lawrence fuming!