DY > There no ona’ah on real estate, and that’s what a room rental is.
Is this so? I did not think of catering as RE transaction? Is the hall the ikar or is the food and service?
You may be right as the bottleneck allowing a fee is due to RE. Thanks for the correction.
As Gadol mentions this could be up to community beis din/Rov to limit such fees. Maybe allow fee for “Chanuka parties” per se, but not for the brisim and bar mitzvas