This is the same argument about “tipping” rebbeim: if people give what they can, from their warmest appreciation, maybe a token gift, and always treating the shadchan or rebbe courteously and speaking well of them, they aren’t entitled to the same full attention as the one who can smear the palms?
And putting myself in a shadchan’s shoes, I might well get more sipuk hanefesh from some of the have-nots and the shidduchim I’d be able to pull off for them.
“Instituting” (note the quotes, I know you haven’t said here it should be required) additional expenses beyond one’s capabilities may cross the lines into achzariyus.