Typically by song. 20 CAD per song. Montreal doesn't, from what I remember charge you to go into the lap dance area. You pay the bar cover but that's it and then it's 20 per song and whatever else depending on what you want with the dancer.
Your direction on how you want to take it but I think it's best to be clear with the dancer what you're thinking/how many songs you want. Or at the very least, have them tell you when a song is up and if a new one is starting.