My fiance and I had the same predicament! it seems not many parties are happening in QC city. I found 3 events that are a potential. There is a ball at the fairmont frontenac and i believe its over 250$ /person. there is a lovely dinner and bank/orchestra at L'Astral in the concorde hotel (this is a beautiful revolving restaurant at the top of the hotel), and the ticket for that is $170/person. what we decided was to go to Largo Resto-club which is a jazz bar, not heavily advertised. I spoke to the owner who was very hospitable over the phone and he has invited a jazz trio to play that night and he is preparing a special menu for the evening at $85/person. he said its quaint approximately 60 person capacity, not a young crowd (teens/rowdiness). We're very happy with our choice.
Good luck to you!