Cheapest fares require 3-week advance purchase and if you are purchasing closer in, that can be a factor. Also, Tue/Wed/Sat travel will generally be lower if you have flexibility. Although we are now in peak summer demand period, so these probably won't help much until the Fall when demand starts to fall off. Other than that, there are roundtrip fares from $231 on IAD-YUL on AC if you are willing to make the drive.