If you have a good/trust worthy travel agent (and willing not to make a lot of profits from you), better ask the staff since they have first hand knowledge about the market. The ticket prices are very different from every airline even though it's the same period. Even today the price is cheap, tomorrow can be totally different. Another way is to watch out for seat sale or on-line deal like priceline.com - especially if you're travelling alone.
Korean Airlines has direct flight from Incheon/S Korea to Toronto (daily?) so it's only 1 stop from HK. However, the ticket is expensive during high seasons because of strong demand for this sector/route - more Korean immigrants to Toronto in the recent years even though KE uses the 747-400 airplane. Japan Airlines may have a stop-over in Vancouver and then connecting using another airline -double check with your travel agent, the price may not be cheap.
Any cheap tickets can usually be found in low seasons.
Reminder! For Toronto, if your flight has a stop-over in any USA city, make sure you have a valid US visa and a a Passport having more than 6 months' validity. Otherwise, you'll not be allowed to continue to Toronto even though it's just a transit/stop over. The US immigration is very strict on this and essentially there's no exception. I heard some travellers simply avoid any US transit/stop-over AT ALL because it's both time consuming/inconvenient and troublesome - all this security/luggage checks, physical search/re-search and questioning this and that. I'll try to avoid US stop-over possible but the problem is all the airlines of my mileage programs have stop-overs at US cities for Toronto-HK flights.