They really don't do yield management, per se, at least on the Canadian. There fares do vary by season, basically on and off, and I think there may be a shoulder season level, but it is pretty set. There are basically two fare tiers within those seasonal rates, a regular discount tier with relatively limited space, and a full fare, plus the Discount Tuesday sales. Off Season fares seem to be about 30% less than high season fares. I didn't do the math just now, that is a guesstimate from memory.
One small note about VIA's definition of 50% off. 50% off is generally 50% off full peak/high season fare, no matter what time of year you are traveling.
Another note, if you are in a position to ride the Canadian about once a year and would like to. A single one-way trip on the Canadian in a private accommodation even at the off-season discount fare or a Discount Tuesday 50% off is enough to get you elite "Privilege" status which starts when you spend more than CAD $999. Privilege members get a coupon worth 50% off of sleeper fares which is GOOD ANY TIME, allowing me to plan ahead. I discovered this by accident when I got the upgraded membership package after having taken a Discount Tuesday trip on the Canadian, having signed up for the frequent traveler program just to get the emails. I have ridden once a year for the last few years because I enjoy it and can afford as long as I long keep those 50% off coupons coming. It isn't a big deal for me to get to Vancouver, I can catch an Amtrak Cascades to Vancouver from a station with free parking 25 minutes from my house. So they have literally hooked me like crack.