Apy,
What is the market rate? If I gave a shadchan $500 and said shadchan took me to beis din because they felt they were entitled to $1000, what would beis din say to me?
That obviously would depend on what they would pasken is the market rate for the community involved.
The definition of community is also something needing psak.
L’maaseh, it rarely, if ever, ends up in beis din, but someone needs to consult with a posek before unilaterally deciding what or how much to give, because it’s a halachic issue and not dependent on personal whim.