I have seen prices of hotels go up and it is very hard to get a room during F1 weekend. I imagine it's harder to book girls from the agencies also. Several years ago I tried booking that weekend 3 weeks in advance and remember the rate was $240 per night at the Delta, and the following weekend the rate was $129 per night. I called the hotel and they told me that it was F1 weekend and they had very few rooms available. So I never been in Montreal during F1 weekend.
The Jazz festival is not the same. You have a lot of people coming in for the races, including fans and participants, like racers, crew, officials, etc. The Jazz Festival runs several weeks, and you have summer vacationers anyhow.