Kayak.com shows flights from Toronto to Paris on July 10, and London to Toronto on July 24 (arbitrary dates) in the $700 range round trip on Aer Lingus, with stops in Dublin, and the $800-900 range on Air Transat, Air Canada, or Lufthansa with no stops. I think those are excellent fares, but I'm used to paying more from the west coast. I'd be very surprised if fares like that were to drop as time passes. More likely they will rise.
I agree with the advice to start early, play around on search engines, get a feel for what's available, and pull the trigger when it feels right. Many search engines and airlines will let you search for dates and/or airports close to the ones you specify. In general, fares will rise as the dates get closer, and waiting for an exception can be frustrating and expensive. When you've found a flight, or airline, that seems to work for you, make the actual purchase on the airline's own website. This will protect you better if you have to change or cancel, and usually costs no more than the fare quoted on the search engine. (If it costs significantly more, then use the engine.)