I was discussing this with a friend just a couple of weeks ago. Here's the math:
There are about 1,600,000 people in Montreal, 800,000 of them female.
Of these 800,000, approximately 100,000 of them are between the ages of 20-30.
If the mean career expectancy of a sexworker is six months (and remember some work just a few days, some work for years) and there are 2,500 sex workers in Montreal at any given time, that means that 50,000 girls work in the sex industry over a 10 year period. If half of them are not from Montreal originally, that reduces the number to 25,000.
If my math is correct, and all of my numbers in the first paragraph are approximate, while those in the second are guesses, the answer to your question would be 25%.
Going just a bit farther now, if you assume that 75% of the female population is not attractive enough to work in the sex industry, what you're left with is 100% of all attractive females in Montreal are, at one time or another, sex workers.
Note that there is the remote possibility that some of my numbers or logic might be off, however slightly.