This may be old news to some, but...
UA has some cheap flights ex SEA (domestic U.S. travel only, I think). Maybe it's just part of their fall seat sale, but prices seemed unusually good. I found SEA-DFW in Nov. for 148 USD = ~208 CAD -- a whopping $312 (CDN) less than flying out of YVR!!
(And that's with an AC seat sale on -- difference would be even more if flying out of YVR on UA.)
Bus from Vancouver to Seattle adds another 60 USD = ~85 CAD return, but even then I'm still ahead about $200 odd CDN. Looks like I'll be spending turkey day in the States...
Not sure what other city pairs will price in cheaply...saw SEA-MIA at around $123 US before taxes. Book in T class, 14 day advance, Sat. night stay required.
Cheers!
ml