Also, I believe the price elasticty of demand for escorts is fairly inelastic. Unless there's really a sudden big price raise, avid hobbiers will keep hobbying. Everyone needs their "fix".
If demand was inelastic the prices would certainly increase in Montreal.
With the exception of a fairly small high end sector that compares a bit to a Veblen good, I'd say that the commercial sex is pretty close to a perfect competition market with lots of producers and buyers (no one can dictate the prices), no barriers to entry and exit and quite good information, thanks to sites like this one. We are far from homogenous services however, so from an economical point of view, we have more then one market, some being substitutes for the others. For some hobbyist, mp's are substitutes for sp's. Lower prices in Montréal, compared to Toronto, and Vancouver are the result of higher budget constraints, lower incomes of the local population.