Montreal has three advantages over Toronto in terms of making these barriers much easier and cheaper to install.
First of all, the platforms aren't cantilevered far over the tracks (where there's a crawl area on the TTC to escape from the train, if you are on the tracks). This makes construction a lot cheaper.
Secondly, Montreal already has had ATC for over 40 years. The trains have been stopping in exactly the same place in the station for so long that you can see where the floor is warn a bit, near the doors!
And the trains already take the full length of the platform in Montreal (9-car trains), so no issues of how the door spacings would change by adding an extra car to use the full platform length.
Should be a lot cheaper there than in Toronto.