WestJet tends to have a lot of seat sales, but I'm not sure what they'll be like in the summer. Their offers tend to be for departures on Tuesdays, Wednesdays and Thursdays.
I flew from Toronto to St. John's in the summer using points for the airfare (and then of course paying the taxes). However for autumn and winter flights, in the end it was a better deal to just book pay a full (cheap) fare and hold onto points for other trips.
Sunwing also has seasonal flights between Toronto and St. John's. Currently there is nothing available between those two points but I know that that flights were available this past autum.
Good luck and happy travels!
~ Cathie ~
Live ♥ Love ♫ Rock"I finally found my way, Say goodbye to yesterday
Hit the gas there ain't no brakes on the lost highway"