You can see the availability of Preferred Seats (and Rouge Plus seats) when you search for the flight on aircanada.com. In the results grid, select a fare and links should appear below the flight numbers to see seats availability for each flights.

To get an idea of the price for preferred seats, have a look at the second page of this document. Take it with a grain of salt as this info is 1.5 years old.

If you book on aircanada.com, you have 24 hours to cancel your purchase and receive a full refund. You can use this to your advantage if you don't like the price for Rouge Plus or if they are no longer available.